Physico-chemical investigation of urolithiasis today is mostly focused on applying physico-chemical description of precipitation processes to the stone formation with the aim to distinguish between stone formers and nonstone formers. This is done by calculating supersaturation for different solid phases which can be formed in urine using data obtained by urine analysis and existing solubility product, dissociation and complexation constants. In order for this approach to succeed it is of utmost importance that system is described as detailed as possible, i.e., that all species that can be formed are taken into account. Magnesium hydrogenurate octahydrate, Mg(C5H3N4O3)28H2O (Mg(HU)2.8H2O), is among species which can precipitate in the urine and for which solubility data doesn't exist. In order to fill this void crystals of Mg(C5H3N4O)2.8H2O phase I and phase II have been prepared and characterized. Solubility product constant of Mg(C5H3N4O3)2.8H2O phase I in water at 37 degrees C and phase II at different temperatures, pH and in different solvents have been determined by measuring total concentration of uric acid and magnesium ions in solutions at different time periods. Results show that in water at 37 degrees C thermodynamically less stable phase I is more soluble (Ksp = (5.64 +/- 0.20).10(-9) mol3 dm(-9)) than phase II (Ksp = (1.66 +/- 0.13).10(-9) mol3 dm(-9)). Solubility of Mg(HU)2.8H2O phase II increases with temperature. At equilibrium the solubility of phase II is the lowest in the presence of excess of magnesium ions, while solubility in the presence of uric acid is comparable with the one obtained in water.