Abstract. For the space of C 3 quintics on the Powell-Sabin 12-split of a triangle, we determine explicitly the six symmetric simplex spline bases that reduce to a B-spline basis on each edge, have a positive partition of unity, a Marsden identity that splits into real linear factors, and an intuitive domain mesh. The bases are stable in the L∞ norm with a condition number independent of the geometry, have a well-conditioned Lagrange interpolant at the domain points, and a quasi-interpolant with local approximation order 6. We show an h 2 bound for the distance between the control points and the values of a spline at the corresponding domain points. For one of these bases we derive C 0 , C 1 , C 2 and C 3 conditions on the control points of two splines on adjacent macrotriangles.