A study of the kinetics and mechanism of the decay of alpha-acetoxy-N-nitrosopyrrolidine and alpha-acetoxy-N-nitrosopiperidine are reported. The compounds differ in reactivity by more than 2 orders of magnitude at physiological pH. On the basis of thermodynamic parameters, common ion inhibition and azide ion trapping experiments, both compounds appear to decompose under these conditions by the formation of N-nitrosiminium ion intermediates; The differences in reactivity are rationalized on the basis of results from an ab initio study, described in the accompanying paper. The first direct study of the kinetics of decay of cyclic alpha-hydroxynitrosamines of nitrosopiperidine and nitrosopyrrolidine and a third compound, 2-hydroxy-2-methylnitrosopyrrolidine is also summarized. These prove to be highly unstable reactive intermediates, in contrast to what might be expected on the basis of earlier reports concerning cyclic alpha-hydroxynitrosamines.