The effect of oxygen on the electrical and optical properties of In0.5Ga0.5P epitaxial layers grown on (100) GaAs by liquid-phase epitaxy has been investigated by adding Ga2O3 to the growth melt. As the amount of Ga2O3 increases, the carrier concentration at 300 K decreases from 4×1016 to 4×1015 cm−3 and the Hall mobility at 77 K increases from 2400 to 4000 cm2/V s. The photoluminescence at 17 K shows that the peak intensity of an extrinsic transition in the In0.5Ga0.5P layer is reduced when Ga2O3 is added to the growth melt. These facts indicate that the main effect of Ga2O3 is the reduction of impurity concentration in the growth melt. In the In0.5Ga0.5P layer grown from the Ga2O3-added growth melt, the same deep trap, with an activation energy of 0.29 eV, as in an undoped layer is observed but the trap density is decreased. This implies that the deep trap is not due to a simple intrinsic defect, but related to an impurity.