In this work high structural and optical quality InxGa1−xP/GaAs quantum wells in a wide range of thicknesses have been successfully grown on GaAs substrates by low temperature atomic layer molecular beam epitaxy. We demonstrate that compositional fluctuations in the barrier alloy are responsible for the inhomogeneous broadening and spatial localization effects observed in the excitonic recombination, the influence of quantum well width fluctuations being negligible in comparison. An important change of the optical transition energies in these quantum wells is observed when tuning a 10% In–Ga ratio in the alloy around the lattice match composition (x=0.48). This change is related to the barrier band gap variation and the intrinsic characteristics of the InGaP/GaAs heterostructure: different exciton binding energy from tensile to compressive strain in the barrier, and a possible dependence of the conduction band offset on the In composition.