Several 1-phosphono-2-aza-1,3-dienes and 1-aryl-1-phosphono-2-aza-1,3-dienes were prepd. by 1,4-dehydrochlorination of the corresponding di-Et [(2-chloro-1-alkylidene)amino]methylphosphonates. 1-Phosphono-2-aza-1,3-dienes react smoothly with diazomethane to give 2-phosphono-1-vinylaziridines. The synthesis of 2-ethoxycarbonyl-3-phosphono-1-vinylaziridines was performed using Et diazoacetate (EDA) in the presence of ytterbium(III) triflate as a catalyst. 2-Phosphono-3-(trimethylsilyl)aziridines were prepd. from phosphonoazadienes by treatment with (trimethylsilyl)diazomethane under reflux. 1-Aryl-1-phosphono-2-aza-1,3-dienes are not susceptible to aziridination under these conditions.