Diphenylphosphine oxide reacts rapidly with azodicarboxylates to give 1 : 1 diphenyl-phosphinoylhydrazine-1,2-dicarboxylate adducts. Structural analysis shows that the diethyl compound crystallises in space group P, a= 12.197(5), b= 9.507(3), c= 8.503(6)Ŭ = 85.83(5), = 76.91 (6), = 75.61(3)ࠡnd the diisopropyl compound crystallises in space group P21/c, a= 21.344(6), b= 9.702(2), c= 10.929(4)Ŭ = 104.71(3)஠The solid state conformations of the two molecules differ. In the diisopropyl structure the nitrogen protons and phosphorus oxygen atoms lie trans to each other whereas in the diethyl structure they are cis. 1H and 13C NMR spectroscopic studies reveal a high degree of hindered rotation. The mechanism of the reaction is discussed. Unexpectedly, diphenylphosphine and diisopropyl azodicarboxylate react slowly to give a complex mixture of products.