Saturation molalities m(sat) in H2O(1) have been measured for the substances cytidine(cr), hypoxanthine(cr), thymidine(cr), thymine(cr), uridine(cr), and xanthine(cr) by using h.p.l.c. The states of hydration were established by performing Karl-Fischer analyses on samples of these substances, which had been allowed to equilibrate with their respective aqueous saturated solutions for several days at T approximate to 298 K and then dried with air at T approximate to 296 K for approximate to 24 h. The crystalline forms of the substances were identified by comparison of the results of X-ray diffraction measurements with results from the literature. Also, molar enthalpies of solution Delta(sol)H(m)(cal) for these substances were measured by using an isoperibol solution calorimeter. A self-association (stacking) model was used to estimate values of the activity coefficients gamma and relative apparent molar enthalpies L-phi for these substances. These gamma and L-phi values were used to adjust the measured values of m(sat) and Delta(sol)H(m)(cal) to the standard state and thus obtain values of the standard molar Gibbs free energy Delta(sol)G(m)degrees and enthalpy changes Delta(sol)H(m)degrees for the dissolution reactions of these substances. The values of the pKs and of the standard molar enthalpies of the ionization reactions were also used to account for speciation of the substances in the calculations of Delta(sol)G(m)degrees and Delta(sol)H(m)degrees. Values of standard molar enthalpies of formation Delta(f)H(m)degrees, standard molar Gibbs free energies of formation Delta(f)G(m)degrees, and standard partial molar entropies S(2,m)degrees for the aqueous species of hypoxanthine and xanthine were calculated. A detailed summary and comparison of thermodynamic results from the literature for these substances is presented. Published by Elsevier Ltd.