Dissolution experiments have been performed in order to determine the solubility constant of coffinite, USiO 4. Several assemblages of phases were used in under-saturated experiments performed in 0.1 mol L-1 HCl under Ar atmosphere, as well as in air. These samples were fully-characterized and were composed of either USiO 4 , solely, or USiO 4 and additional oxide byproducts that resulted from the synthesis procedure. The solubility constant of coffinite was determined at 25°C and 1 bar (log *K S°(USiO 4 , cr) =-5.25 ± 0.05), as well as the standard free energy of formation of coffinite (Δ f G°(298 K) =-1867.6 ± 3.2 kJ mol-1), which enables one to infer the relative stability of coffinite and uraninite as a function of groundwater composition. Geochemical simulations using PHREEQC 2 software and the Thermochimie data base indicate that coffinite precipitates at 25°C under reducing conditions, at pH = 6, for H 4 SiO 4 (aq) concentration of 7 10-5 mol L-1 and U(OH) 4 (aq) concentration of 10-11 mol L-1. The Δ f G°