A reinforcing agent for vulcanizable rubber compounds comprises an anhydrous metal salt of an .alpha.,.beta.-ethylenically unsaturated carboxylic a cid having a crystalline structure consisting essentially of plates and fibers. Vulcanizable rubber compounds containing such reinforcing agents have improved physical properties over rubber compounds containing conventional metal carboxylic acid salts. Methods for the preparation of such anhydrous metal salts and methods for the reinforcement of vulcanizable rubber compounds therewith are also provided.