Relative thermodynamic stabilities in two series of isomeric complexes of substituted pyrimidine (4-aminopyrimidine; 4-hydroxypyrimidine, 4-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. In each of the sets several relative stability interchanges are found. The interchanges are manifested in a considerable heat capacity term enhancement.