Tautomerisms of N1-methyl, N3-methyl, and N1, N3-dimethyl derivatives of uracil, 5-fiuorouracil and thymine have been examined in the gas phase and in water. Geometry optimizations were carried out at the HF/6-31G**. HF/6-31 divided by G** and B3LYP/6-31 divided by G** levels. Also. single-point MP2/6-31 + G** calculations were performed on the HF/6-31 +G** optimized geometries. The influence of the solvent was examined from the self-consistent reaction field (SCRF) calculations. Analyzing the results, only in the case of attachment of fluorine atom at position 5 of N1-methyl uracil changes the order of the stabilities of the tautomers. (C) 2004 Elsevier B.V. All rights reserved.