In order to obtain 4nitrodiphenylarsinic acid, phenylarsine oxide was allowed to react with diazotized 4-nitroaniline. The nitro acid was reduced to the corresponding amino acid and the latter converted into 4-hydroxydiphenylarsinic acid by the diazo reaction. Treatment of the latter compound with hypophosphorous acid yielded 4, 4”-dihf. droxytetraphenyldiarsyl.