The preparation of Th1-xUxSiO4 uranothorite solid solutions was successfully undertaken under hydrothermal conditions (T = 250 degrees C). From XRD and EDS characterization, the formation of a complete solid solution between x = 0 (thorite) and x = 0.8 was evidenced. Nevertheless, additional (Th,U)O-2 dioxide and amorphous silica were systematically observed for the highest uranium mole loading. The influence of kinetics parameters was then studied to avoid the formation of such side products. The variation of the synthesis duration allowed us to point out the initial formation of oxide phases then their evolution to a silicate phase through a dissolution/precipitation process close to that already described as coffinitization. Also, the uranium mole loading initially considered was found to significantly influence the kinetics of reaction, as this latter strongly slows down for x > 0.3. Under these conditions, the difficulties frequently reported in the literature for the synthesis of pure USiO4 coffinite were assigned to a kinetic hindering associated with the coffinitization reaction.