In a constant ionic medium, corresponding to a physiological environment (I-c = 0.15 mol dm(-3) NaCl), the solubilities of anhydrous uric acid, uric acid dihydrate and monosodium urate monohydrate have been measured as a function of p[H] = -log [H+](2-8) and temperature (25 degrees, 32 degrees, 37 degrees and 42 degrees C). The solubility equilibria in the uric acid-sodium urate-water system are discussed on the basis of the solubility constants (K-s) and the first dissociation constant (K-1) of uric acid and the solubility product (K-s0) of monosodium urate. The quantities measured in this work are in good agreement with literature values, however, the present solubility data have a much higher precision.