Relative stabilities in two pairs of complexes of tautomers of substituted pyrimidine (2-aminopyrimidine and 2-iminopyrimidine; 2-hydroxypyrimidine and 2-oxypyrimidine) with water have been computed. Description of the isomers is based on ab initio SCF calculation, with energetics corrected by the second-order perturbation treatment. The relative stabilities are evaluated in terms of partition functions constructed from the calculated dimeric parameters. Complexes of 2-hydroxypyrimidine and 2-oxypyrimidine with water were found to coexist at higher temperatures. However, the overall heat capacity can be enhanced by about 3%, at most, by the isomeric interplay. The relation to non-equilibrium situations is discussed.