A comprehensive theoretical treatment of the factors governing the solubility of uric acid and monosodium urate is given. The various constants appearing in the treatment are evaluated as functions of temperature and ionic strength using both existing data and new experimental data. The solubility of uric acid increases with increasing pH, while the solubility of monosodium urate has a minimum near pH 7·7 at 37°C. The solubility product of monosodium urate increases with increasing ionic strengt