Multiparametric study of the hydrothermal synthesis of thorite, ThSiO4, was performed with the aim to determine the most efficient conditions to form single phase thorite samples. Among the experimental parameters investigated, temperature of the hydrothermal process, concentration of carbonate ions, thorium and silicon reactants, and pH of the reactive media significantly affect the composition of the final system obtained. Single phase samples of ThSiO4 were prepared in weakly basic reactive media and at temperatures over 150 °C, for thorium and silicate concentrations higher than 8 × 10-3 mol L-1 and carbonate concentrations of at least 8 × 10-2 mol L-1. Although the synthesis of thorite in carbonate media was already described in the literature, this study gives new insights to explain the key role of carbonate ions in the preparation of thorite. Especially, beyond their simple role of pH buffer, carbonate ions are involved in the formation of thorium-carbonate complexes at high pH, increasing the apparent solubility of thorium in weakly basic media. The presence of carbonate ions has an important impact not only on the domain of formation of thorite but also on the morphology of the silicate phase.