The carbonyl carbon, carbonyl oxygen, and nitrogen nucleophile isotope effects were measured for the hydrazinolysis of methyl formate at pH 8 and 10. At pH 8, where breakdown of a tetrahedral intermediate to products is rate-determining, the carbonyl carbon isotope effect is k(12)/k(13) = 1.038, the carbonyl oxygen isotope effect is k(16)/k(18) = 1.003, and the nitrogen nucleophile isotope effect is k(14)/k(15) = 0.990. The isotope effects at pH 8 are consistent with a late transition state, which greatly resembles the hydrazide product. At pH 10, where formation of a tetrahedral intermediate is rate-determining, the carbonyl carbon isotope effect is k(12)/k(13) = 1.020, the carbonyl oxygen isotope effect is k(16)/k(18) = 1.004, and the nitrogen nucleophile isotope effect is k(14)/k(15) = 0.9917. These isotope effects are best rationalized in terms of a concerted general base catalyzed nucleophilic attack of hydrazine on methyl formate as the rate-determining step.