Dephosphorylation catalyzed by a base occurs in nucleoside 5'-phosphorami dates (1a-3b) and nucleoside 5'-thiophosphoramidates (4a-5b). The reaction rates for both classes of compounds increase with the basicity of the catalysts in the solution. The kinetics and mechanisms of the reaction were investigated by NMR spectroscopy and a semi-empirical quantum mechanics method. Our experimental results showed that the reaction was pseudo first order. The rate constants of the dephosphorylation for compounds 1a-3b were faster than those for compounds 4a-5b. The effects of the structures of compounds on the reaction were discussed. (c) 2005 Elsevier B.V. All rights reserved.