Epitaxially grown lattice-matched GaInP on a GaAs crystal is a common part in semiconductor devices such as bipolar junction transistors and space solar cells. Due to the larger band gap of GaInP, it provides also high-quality passivation for GaAs surfaces. Therefore, the photoluminescence (PL) intensity measured from GaInP-capped GaAs is among the strongest intensities obtained from GaAs crystals having different surface passivation layers. Here we demonstrate that a facile wet chemical treatment, including immersions in two solutions: first in hot hydrogen peroxide (H₂O₂) and then in hot water (H₂O), increases the PL intensity from epitaxial GaInP/GaAs. Concomitantly, the GaInP surface is further oxidized according to x-ray photoelectron spectroscopy results. Particularly, arsenic impurities and indium at the surface become oxidized. Finally, the H₂O₂ → H₂O treatment combination is used to modify the n-type GaAs contact layer (150 nm), deposited on the top of epitaxial GaInP/GaAs such that the PL intensity from GaAs increases by factor of two as compared to PL from the high-quality GaInP/GaAs reference. The H₂O₂ → H₂O treatment is discussed to transform an initial n-type GaAs contact layer, which degrades the PL intensity, to an antireflective and less absorptive layer which resembles a recently reported black GaAs surface.