A series of thymidine and tetrahydrofurfuryl phosphoramidates bearing haloethyl or piperidyl substituents was synthesized and used to investigate the activation mechanisms of nucleoside phosphoramidate prodrugs. Structure assignments for the tetrahydrofurfuryl reaction products were confirmed by comparison to authentic samples. Structural assignments for thymidine phosphoramidate reaction products were made by analogy to the tetrahydrofurfuryl products. Generation of the phosphoramidate anion leads to cyclization and subsequent nucleophilic attack at carbon and phosphorus of the resulting aziridinium ion intermediate to give the observed products. Nucleophilic attack by water at carbon and phosphorus occurs without selectivity, supporting a mechanism of action of haloethylamine nucleoside prodrugs involving intracellular release of the nucleotide. Activation of the benzotriazolyl piperidyl phosphoramidates is followed by P-N bond hydrolysis; this reaction is subject to specific acid catalysis and to nucleophilic catalysis by 1-hydroxybenzotriazole. These results suggest that the mechanism of action of the piperidyl nucleoside phosphoramidates involves the intracellular release of the active nucleotide following P-N bond cleavage, presumably by the action of an endogenous phosphoramidase.