A light stable polyurethane elastomer is prepared in the absence of solvent from a polyol, a chain extender and an aliphatic polyisocyanate using a thio tin organic compound, containing sulfur directly attached to the tin, as a heat activated catalyst. E.g., a solventless liquid composition comprising an hydroxy-terminated polyester, a chain extender, and hydrogenated MDI, containing dibutyltin S,S-bis(isooctyl mercaptoacetate) as a catalyst, has a pot life in excess of 25 minutes at room temperature but upon heating 1 minute at 375.degree.F. gels to a solid state. Because of this combination of long pot life and rapid cure, the composition is suitable for such applications as making coated fabrics and films, and in other liquid processing techniques, e.g., rotational casting, liquid injection molding, and the like.