This invention relates' to polyurethane/polyureas with improved catalysis and to a process for the preparation of these polyureihane/polyureas. These materials, comprise the reaction product of (A) a (cyclojaliphatic polyisocyanate or (cyclo)aliphatic polyisocyanate prepolymer, with (B) an isocyanate-reactive component comprising at least one aromatic diamine compound containing from 1 to 8 thiomethyl groups, in the presence of (C) one or more catalysts selected from the group consisting of tin (II) catalysts, bismuth (III) catalysts and mixtures thereof. Preferably, the materials are optically clear.