The phase diagrams and band gaps in CuInSe2-CuGaSe2 (CIS-CGS) and CuInSe2-CuAlSe2 (CIS-CAS) pseudobinary systems are determined using a combination of first-principles calculations based on a hybrid Hartree-Fock density functional approach, cluster expansion, and Monte Carlo simulations. The CIS-CGS and CIS-CAS systems show phase-separation critical temperatures of 420 and 460 K, respectively. For both CuIn1−xGaxSe2 (CIGS) and CuIn1−xAlxSe2 (CIAS) alloys, the dependence of the band gaps on the temperature before quench is suggested to be small through the analysis of atomic configurational effects. The band gaps are generally close to those at the ideal disorder state and exhibit nearly quadratic composition dependences, i.e., band bowing. Composition-dependent bowing behavior is identified for CIAS, with a stronger bowing in a higher CAS composition range.