A series of N-carbamoyl aziridines has been treated by diethyl phosphite in presence of n-BuLi to afford α-methylene phosphonate aziridines in modest yields. The study of the reaction's scopes and the analysis of byproducts indicated that this transformation proceeds via a unique mechanism. The mechanism that produces the α-methylene phosphonate relies in the use of BuLi, where both the lithium ion and then the presence of the nucleophilic butyl is essential. In addition, the nature of the final compound, either α-methylene phosphonate or α-methylene-gem-bisphosphonate derivatives containing an aziridine motif, is highly dependent on the nature of the base used.