The Th-Pb system was investigated using differential thermal, X-ray and metallographic analyses. As for the Th-Sn system, four intermediate phases were found, two of which (Th 5Pb 3 and Th 5Pb 4) were not previously known. The existence of the two compounds ThPb 2 and ThPb 3, was confirmed but no evidence was found for the formation of ThPb and ThPb 4 which have been reported in the literature. One eutectic point at about 1.0 at.% Th with a melting temperature of 310°C was determined. ThPb 3 and ThPb 2 were formed by peritectic reactions at 1100°C and 1230°C respectively. The diagram for compositions with a thorium content greater than that of ThPb 2 was proposed on the basis of a survey of other known R-Pb diagrams. A brief analysis of the existence and crystal structures of intermetallic compounds of thorium with group IVb elements is also reported.