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Results are presented from a detailed study of the P-N bond hydrolysis in guanosine 5-prime-monophosphate 2-methylimidazolide (2-MeImpG) and in guanosine 5-prime-imidazolide (ImpG) in the presence of 0-0.50 M Mg(2+). Pseudo-first-order rate constants of these compounds were obtained as a function of Mg(2+) concentration, for pH values between 6 and 10 and 37 C. It was found that Mg(2+) catalysis was most effective at pH 10, where a 15-fold increase in hydrolysis was achieved in 0.02 M Mg; at 0.2 M, a 115-fold increase was observed. Implication of these results for the mechanism of template-directed oligomerization is discussed.