Interaction in the CuInSe₂-Pb-Se quasi ternary system were studied by DTA, X-ray powder diffraction, microstructural analysis and microhardnes measurements. A projection of the liquidus surface of the phase diagram was constructed, where the boundaries of the primary crystallization region of the low-temperature chalcopyrite phase were determined. The existence of the compound with a rational formula CuInPbSe₂ was established, which forms at 650⁰C by a peritectic reaction