Compositional effects on optical band-gap energy using end members of ABO3 perovskites have been investigated through an optical absorption with a UV-spectroscopy. Three examples are selected, namely, BaTiO3–CaTiO3, BaTiO3–BaZrO3, and SrTiO3–BaZrO3 solid solutions. To understand the role of high temperature phase equilibria on the band-gap compositional trends, structural and microscopy data were determined. In simple systems such as Si–Ge, the nonlinear variations in band gap with composition is usually associated with the effect of the local lattice relaxations and provides a parabolic dependence, often referred to as the “bowing” phenomena. In the case of perovskite solutions, the cases are more complex, and a modified Vegard’s law is introduced to account for the trends. This has to be considered in relation to high temperature phase formation where incomplete solid solutions and two-phase regions exist. In addition to high temperature phases, low temperature displacive phase transitions and complex nonstoichiometry also perturb the band-gap variation in perovskite oxide materials.