We investigated the protomeric tautomerism of N-vinyl-2-pyrrolidinone (N-V2P) and N-vinyl-3-pyrrolidinone (NV3P) molecules, and considered the three neutral tautomers for NV2P (1a, 1b, 1c) and the five for NV3P (2a, 2b, 2c, 2d, 2e). Full geometry optimizations were carried out at the HF/6-31G**, HF/6-31+G** and B3LYP/6-31+G** levels in the gas phase and in water. Additionally, single-point MP2/6-31+G** calculations were performed on the HF/6-31+G** optimized geometries. The tautomerization processes in water (epsilon = 78.54) were studied using the self-consistent reaction field theory. According to our estimations, the tautomerization of NV2P and NV3P molecules are not strongly influenced by the polar solvents, the improvement of the basis set quality and the electron correlation effects in the gas phase and in water. The calculated relative free energies (DeltaG) predict that 1a and 2a are the energetically preferred tautomers in the gas phase and in water. (C) 2001 Elsevier Science B.V. All rights reserved.