Abstract The Th-Sn system was investigated by means of differential thermal and X-ray analysis and micrography. Two eutectics are formed at about 80.0 at.% Th and 1.0 at.% Th with characteristic temperatures of 1600 °C and 220 °C respectively. Four intermetallic compounds (Th5Sn3, Th5Sn4, ThSn2 and ThSn3) were found and the crystal structures of all except ThSn3 were determined using single-crystal diffractometric data. Th5Sn3 melts congruently at 1800 °C, whereas ThSn2 and ThSn3 decompose peritectically at 1410 °C and 955 °C respectively. It was impossible to determine the decomposition point of Th5Sn4 and the temperature reported in the diagram (1550 °C) is an estimated value. No appreciable solid solubility of thorium in tin or of tin in thorium was observed; the four intermediate phases also appear to be line compounds.