The decay of alpha-(acyloxy)dialkylnitrosamines in aqueous solutions has been studied with a view toward elucidating mechanistic details and effects of structure on mechanism and reactivity. Rate constants (k(1)) for the pH-independent decay of 43 alpha-(acyloxy)dialkylnitrosamines have been determined. Observations from these and other experiments rule out decomposition via an anchimeric assistance mechanism involving the Z isomer that had previously been suggested. All of the reported data for most of the compounds is consistent with a mechanism involving the formation of N-nitrosiminium ions in or before the rate-limiting step. Structure reactivity correlations indicate that the stability of alpha-(acyloxy)dialkylnitrosamines is determined by electronic properties of substituents at R-N and R-C as well as by the ability of substituents R-C to engage in hyperconjugative interactions of C-H bonds with the developing cationic center in the transition state for nitrosiminium ion formation. Attachment of substituents of sufficient electron-withdrawing power at R-N and R-C results in a predicted change in mechanism to what appears to be an acyl group attack mechanism.