The reactions of N-amidino-O-alkylisourea and 1-substituted 3-amidino-2-thiourea with benzil were examined. The reaction of N-amidino-O-alkylisourea with an equivalent amount of benzil in ethanol gave 5, 5-diphenylglycocyamidine by heating for 10 hr under reflux, 5, 5-diphenylglycocyamidine and 2-alkoxyamidinylidene-5, 5-diphenylhydantoin by heating for 2 hr under the similar conditions and 2-alkoxyamidinylidene-5, 5-diphenylhydantoin and 3a, 6a-diphenylglycoluril-2, 5-dialkoximidoylimide by treatment for 1 hr at room temperature. Similarly, heating of 1-aryl-3-amidino-2-thiourea with benzil in ethanol for 10 hr under reflux gave 5, 5-diphenylglycocyamidine. On the other hand, by heating in ethanol in the presence of potassium hydroxide for 2 hr under reflux, 2-arylthiocarbamoyl-5, 5-diphenylglycocyamidine.