Abstract: This invention is directed to an improved process for preparing 2-phenylethylene phosphonic acid by reacting styrene and PCl.sub.5 in an inert organic solvent and hydrolyzing the reaction product formed, wherein the improvement comprises forming a mixture of styrene and PCl.sub.5 in a molar ratio of about 1:1 at a temperature of from 60.degree. to 70.degree. C. in an inert organic solvent having a boiling point above 70.degree. C.; maintaining the mixture at a temperature of from 60.degree. to 70.degree. C. with evolution of HCl gas until a clear solution forms; distilling off the solvent under vacuum; hydrolyzing the remaining reaction mixture in water at temperatures not exceeding 70.degree. C. and cooling the hydrolyzed mixture to cause crystals of 2-phenylethylene phosphonic acid to precipitate; and recovering said crystals.