diff --git a/Kernel_23/include/CGAL/Direction_2.h b/Kernel_23/include/CGAL/Direction_2.h
index be601005d9e..3b6fbd9d8a9 100644
--- a/Kernel_23/include/CGAL/Direction_2.h
+++ b/Kernel_23/include/CGAL/Direction_2.h
@@ -70,16 +70,16 @@ public:
   Direction_2(const RDirection_2& d)
     : RDirection_2(d) {}
 
-  Direction_2(const Vector_2& v)
+  explicit Direction_2(const Vector_2& v)
     : RDirection_2(typename R::Construct_direction_2()(Return_base_tag(), v)) {}
 
-  Direction_2(const Line_2& l)
+  explicit Direction_2(const Line_2& l)
     : RDirection_2(typename R::Construct_direction_2()(Return_base_tag(), l)) {}
 
-  Direction_2(const Ray_2& r)
+  explicit Direction_2(const Ray_2& r)
     : RDirection_2(typename R::Construct_direction_2()(Return_base_tag(), r)) {}
 
-  Direction_2(const Segment_2& s)
+  explicit Direction_2(const Segment_2& s)
     : RDirection_2(typename R::Construct_direction_2()(Return_base_tag(), s)) {}
 
   Direction_2(const RT &x, const RT &y)
diff --git a/Kernel_23/include/CGAL/Direction_3.h b/Kernel_23/include/CGAL/Direction_3.h
index 776af50ceda..f0c1b3a3958 100644
--- a/Kernel_23/include/CGAL/Direction_3.h
+++ b/Kernel_23/include/CGAL/Direction_3.h
@@ -69,16 +69,16 @@ public:
   Direction_3(const Rep& d)
     : Rep(d) {}
 
-  Direction_3(const Vector_3& v)
+  explicit Direction_3(const Vector_3& v)
     : Rep(typename R::Construct_direction_3()(Return_base_tag(), v)) {}
 
-  Direction_3(const Line_3& l)
+  explicit Direction_3(const Line_3& l)
     : Rep(typename R::Construct_direction_3()(Return_base_tag(), l)) {}
 
-  Direction_3(const Ray_3& r)
+  explicit Direction_3(const Ray_3& r)
     : Rep(typename R::Construct_direction_3()(Return_base_tag(), r)) {}
 
-  Direction_3(const Segment_3& s)
+  explicit Direction_3(const Segment_3& s)
     : Rep(typename R::Construct_direction_3()(Return_base_tag(), s)) {}
 
   Direction_3(const RT& hx, const RT& hy, const RT& hz)
diff --git a/Kernel_23/include/CGAL/Line_2.h b/Kernel_23/include/CGAL/Line_2.h
index 5302f482ab7..2cce6a37365 100644
--- a/Kernel_23/include/CGAL/Line_2.h
+++ b/Kernel_23/include/CGAL/Line_2.h
@@ -77,10 +77,10 @@ public:
   Line_2(const RT &a, const RT &b, const RT &c)
     : RLine_2(typename R::Construct_line_2()(Return_base_tag(), a,b,c)) {}
 
-  Line_2(const Segment_2& s)
+  explicit Line_2(const Segment_2& s)
     : RLine_2(typename R::Construct_line_2()(Return_base_tag(), s)) {}
 
-  Line_2(const Ray_2& r)
+  explicit Line_2(const Ray_2& r)
     : RLine_2(typename R::Construct_line_2()(Return_base_tag(), r)) {}
 
   Line_2(const Point_2 &p, const Direction_2 &d)
diff --git a/Kernel_23/include/CGAL/Line_3.h b/Kernel_23/include/CGAL/Line_3.h
index a4a89be174b..9f1bc642893 100644
--- a/Kernel_23/include/CGAL/Line_3.h
+++ b/Kernel_23/include/CGAL/Line_3.h
@@ -75,10 +75,10 @@ public:
   Line_3(const Point_3 & p, const Point_3 & q)
       : Rep(typename R::Construct_line_3()(Return_base_tag(), p, q)) {}
 
-  Line_3(const Segment_3 & s)
+  explicit Line_3(const Segment_3 & s)
       : Rep(typename R::Construct_line_3()(Return_base_tag(), s)) {}
 
-  Line_3(const Ray_3 & r)
+  explicit Line_3(const Ray_3 & r)
       : Rep(typename R::Construct_line_3()(Return_base_tag(), r)) {}
 
   Line_3(const Point_3 & p, const Direction_3 & d)
diff --git a/Kernel_23/include/CGAL/Plane_3.h b/Kernel_23/include/CGAL/Plane_3.h
index 75840e7f063..a4e1e1aa09f 100644
--- a/Kernel_23/include/CGAL/Plane_3.h
+++ b/Kernel_23/include/CGAL/Plane_3.h
@@ -93,7 +93,7 @@ public:
   Plane_3(const Ray_3& r, const Point_3& p)
     : Rep(typename R::Construct_plane_3()(Return_base_tag(), r, p)) {}
 
-  Plane_3(const Circle_3& c)
+  explicit Plane_3(const Circle_3& c)
     : Rep(typename R::Construct_plane_3()(c)) {}
 
   Plane_3 transform(const Aff_transformation_3 &t) const
diff --git a/Kernel_23/include/CGAL/Sphere_3.h b/Kernel_23/include/CGAL/Sphere_3.h
index ed4750fbe12..b5e50220981 100644
--- a/Kernel_23/include/CGAL/Sphere_3.h
+++ b/Kernel_23/include/CGAL/Sphere_3.h
@@ -84,10 +84,10 @@ public:
            const Orientation& o = COUNTERCLOCKWISE)
    : Rep(typename R::Construct_sphere_3()(Return_base_tag(), p, q, o)) {}
 
-  Sphere_3(const Point_3& p, const Orientation& o = COUNTERCLOCKWISE)
+  explicit Sphere_3(const Point_3& p, const Orientation& o = COUNTERCLOCKWISE)
    : Rep(typename R::Construct_sphere_3()(Return_base_tag(), p, o)) {}
 
-  Sphere_3(const Circle_3& c)
+  explicit Sphere_3(const Circle_3& c)
    : Rep(typename R::Construct_sphere_3()(c)) {}
 
   Sphere_3 orthogonal_transform(const Aff_transformation_3 &t) const;
diff --git a/Kernel_23/include/CGAL/Vector_2.h b/Kernel_23/include/CGAL/Vector_2.h
index 21b51f040f0..1419ce25ed0 100644
--- a/Kernel_23/include/CGAL/Vector_2.h
+++ b/Kernel_23/include/CGAL/Vector_2.h
@@ -78,13 +78,13 @@ public:
   Vector_2(const Point_2& a, const Point_2& b)
       : RVector_2(typename R::Construct_vector_2()(Return_base_tag(), a, b)) {}
 
-  Vector_2(const Segment_2 &s)
+  explicit Vector_2(const Segment_2 &s)
       : RVector_2(typename R::Construct_vector_2()(Return_base_tag(), s)) {}
 
-  Vector_2(const Ray_2 &r)
+  explicit Vector_2(const Ray_2 &r)
       : RVector_2(typename R::Construct_vector_2()(Return_base_tag(), r)) {}
 
-  Vector_2(const Line_2 &l)
+  explicit Vector_2(const Line_2 &l)
       : RVector_2(typename R::Construct_vector_2()(Return_base_tag(), l)) {}
 
   Vector_2(const Null_vector &v)
diff --git a/Kernel_23/include/CGAL/Vector_3.h b/Kernel_23/include/CGAL/Vector_3.h
index ccc9b5d5756..2c12ba972ba 100644
--- a/Kernel_23/include/CGAL/Vector_3.h
+++ b/Kernel_23/include/CGAL/Vector_3.h
@@ -77,13 +77,13 @@ public:
   Vector_3(const Point_3& a, const Point_3& b)
     : Rep(typename R::Construct_vector_3()(Return_base_tag(), a, b)) {}
 
-  Vector_3(const Segment_3& s)
+  explicit Vector_3(const Segment_3& s)
     : Rep(typename R::Construct_vector_3()(Return_base_tag(), s)) {}
 
-  Vector_3(const Ray_3& r)
+  explicit Vector_3(const Ray_3& r)
     : Rep(typename R::Construct_vector_3()(Return_base_tag(), r)) {}
 
-  Vector_3(const Line_3& l)
+  explicit Vector_3(const Line_3& l)
     : Rep(typename R::Construct_vector_3()(Return_base_tag(), l)) {}
 
   Vector_3(const Null_vector& v)
diff --git a/Kernel_23/test/Kernel_23/include/CGAL/_test_cls_circle_3.h b/Kernel_23/test/Kernel_23/include/CGAL/_test_cls_circle_3.h
index 9cf00e56f4a..267f0f603e5 100644
--- a/Kernel_23/test/Kernel_23/include/CGAL/_test_cls_circle_3.h
+++ b/Kernel_23/test/Kernel_23/include/CGAL/_test_cls_circle_3.h
@@ -144,8 +144,8 @@ void _test_circle_construct(const K &k) {
     assert(theEqual_3(circle,circle2));
     assert(theEqual_3(circle,circle3));
 
-		Plane_3 pus = circle2;
-		Sphere_3 sus = circle3;
+		Plane_3 pus(circle2);
+		Sphere_3 sus(circle3);
 		assert(pus == circle2.supporting_plane());
 		assert(sus == circle3.diametral_sphere());
   }