A reinvestigation of these systems has indicated the marked effect of the purity of materials on the phases formed. New polychalcogenides have been found (ThS2.5, and ThSe2.5 having tetragonal symmetry, and ThTe3 monoclinic), as also has a sesquitelluride (Th2Te3, with hexagonal symmetry) which is probably non-stoichiometric. Discrepancies between the present results and those of previous workers are discussed.