A comparative study of the crystal structure and band gap of the 1:1:2, 1:3:5, and 1:5:8 phases of Cu-In-Se, Cu-Ga-Se, CuIn-Te, and Cu-Ga-Te ternary systems has been made. Attempt is made to explain the observed change in the band gap of these systems due to the shift of the conduction band minimum and valence band maximum caused by the p - d interband repulsion originating from the (III Cu 2+ + 2 V Cu - ) defect pair.