Phenylmercuric propionate is a more active catalyst for isocyanate-hydroxyl reactions than conventional catalysts. For example, dibutyltin dilaurate, stannous octoate, zinc octoate, triethylenediamine, and N-methyl morpholine, gives polyurethane films and urethane rubber having significantly better properties than those produced using the conventional catalysts. Molecular Formula: C9H10HgO2 Molecular Weight: 350.76