At a constant ionic strength corresponding to human urine (I-c=0.300 mol.dm(-3)), the solubilities of xanthine were measured as a function of -Ig{c(H+)/c(0)} (c(0)=1 mol.dm(-3)) at the temperatures T = 298.15 K and T = 310.15 K, respectively. Highly reproducible solubility and dissociation constants were obtained. Also, for the first time, the dissolution enthalpy of xanthine was determined calorimetrically. The values of this quantity obtained from both calorimetric determination and temperature dependence of solubility equilibrium constants are thermodynamically consistent. (C) 2001 Academic Press.