Phosphoric acid diester 2,5-dioxo-4,4-diphenylimidazolidin-1-ylmethyl esters, both of whose ester groups can be selectively cleaved, are obtained by converting 3-(hydroxymethyl)-5,5-diphenylimidazolidine-2,4-dione to an alkylsulfonate or arylsulfonate and reacting this with a phosphoric acid diester whose ester groups can be selectively cleaved from the reaction product.The two ester groups can be selectively cleaved from the phosphoric acid diesters obtained and the resulting product can be converted to 5,5-diphenyl-3-[(phosphonooxy)methyl]imidazolidine-2,4-dione disodium salt. The latter is an anticonvulsive, antiepileptic and antiarrhythmic known under the abbreviated name of sodium fosphenytoin.