Epitaxial layers of InP and InGaAsP have been grown on (100) InP substrates by gas source molecular beam epitaxy over the temperature range 400–480 °C while simultaneously exposed to an Ar plasma stream produced by electron cyclotron resonance (ECR). Transmission electron microscopy, x-ray diffraction, and photoluminescence studies indicate improved structural and optical properties of the InGaAsP layers as compared to layers grown by conventional epitaxy without plasma. This improvement is attributed to a reduction in lateral composition modulation (LCM), which develops at the surface during growth due to the existence of a miscibility gap. Comparison of these results with that achieved by an independent thermal hydrogen cracker suggests that the reduced LCM results from molecular hydrogen, produced from the cracking of the group V hydride sources, backflowing into the ECR chamber and resulting in a flux of atomic hydrogen toward the growth front. Atomic hydrogen exposure of the growing surface may then result in surfactant-mediated epitaxy, thereby, reducing the adatom surface diffusion length and, hence, the LCM. Atomic hydrogen, therefore, appears to be the sole actor in reducing the LCM, while the effects of the plasma itself are negligible.