Abstract Phase equilibria in the Se-rich part of the Pb–In–Se system have been investigated by metallographic examinations, DTA, DSC, XRD and EDX measurements. Two ternary phases were observed: S (Pb 1.46In 2.86Se 5.71) and B′ (Pb 0.8In 3.5Se 5.7). The S compound was found to form congruently, while the B′ compound forms incongruently. Reactions of alloys crystallisation are determined for the Se–rich part. Miscibility gaps which emerge from the In–Se and Se–Pb binary systems extend into the ternary system to form one gap across it. Reaction scheme for the PbSe–Se–InSe system is proposed.