The complexation of cytosine with one water molecule was studied by quantum chemistry. The three lowest-energy tautomers, with three water binding positions for each, were considered and relative stabilities determined. MP2 geometries and CCSD(T) energies were calculated, using basis sets up to aug-cc-pVQZ. DFT calculations are also included. For all three tautomers, the favorite binding position of water is the O=C-2-N-1-H (H-O-C-2=N-1 in the enol form) moiety. Water binds to amino-keto 1.5-2.0 kcal/mol stronger than to the enol form and this roughly compensates for the energy difference in free monomers. Our methodological conclusion is that highest-level (coupled cluster) calculations should be used for the monomers, while MP2 or DFT calculations are satisfactory for describing the hydration itself. This way, our estimate for the relative energies of the complexes with respect to the amino-enol form is 0.0 to -0.5 kcal/mol for the amino-keto and 0.6-1.2 kcal/mol for imino-keto forms. (C) 2002 Published by Elsevier Science B.V.