Kinetic isotope effects of nitrogen and hydrogen on thermolysis of O-ethyl N-mesityl- (1) and O-ethyl N-phenylphosphoramidates (2) in anhydrous ethanol at 80 degrees C are reported. The solvent hydrogen effect k(EtOH)/k(EtOD) is equal to 0.62 +/- 0.03 for 1 and 0.67 +/- 0.05 for 2. The kinetic nitrogen isotope effect k(14)/k(15) was found to be equal to 1.0056 +/- 0.0011 for 1 and 0.9925 +/- 0.0024 for 2. These results are consistent with a mechanism which involves proton transfer from the OH group to the nitrogen moiety. The recalculated nitrogen kinetic effect for the second step (amine departure) is equal to 1.021 for 1 and 1.008 for 2. On this basis a metaphosphate-like transition state or "exploded" S(N)2(P) transition state is proposed for solvolysis of 1. With its less bulky substituent, an earlier transition state in the unimolecular process or a change into the addition-elimination mechanism is considered for phosphoramidate 2.