Syntheses of 5-phosphono-2-oxazolidinones and 5-phosphono-2-imidazolidinones were achieved from the corresponding 1-vinyl-2-phosphonoaziridines. Regioselective aziridine ring opening employing Me chloroformate affords 1-amido-2-chloroethylphosphonates, which were easily transformed into the corresponding 2-oxazolidinones upon heating in DMSO. Treatment of the aziridine ring opening products with ammonia furnishes vinylphosphonates, which undergo a Michael type addn. with several amines. In situ ring closure of the addn. products yields the corresponding phosphonylated 2-imidazolidinones.