The growth of InAsyP1−y onto (100) InP by gas-source molecular beam epitaxy was examined systematically, focusing on control of the resulting As/P incorporation ratio. The group V fluxes were obtained by passing phosphine and arsine through a dual-input low-pressure gas cracker. For a given flow ratio of the source gases, the arsenic fraction y of the resulting InAsyP1−y films is seen to increase with the film thickness over the first 1500 Å (1 Å = 10−10 m) as indicated by secondary ion mass spectroscopy, Auger depth profiling, and by Rutherford backscattering spectroscopy. Thin, strained InAsyP1−y layers (0.30 < y < 0.70, corresponding to a compressive strain of about 1.0–2.2%) contain about 5–20% less As than similarly grown thicker, relaxed layers. For a given growth rate and substrate temperature, the relative compositional shift is found to be linearly proportional to the effective strain corresponding to y. Substrate temperatures above 475 °C further reduce the incorporation ratio of As into both strained and relaxed InAsyP1−y layers, initially enhancing the strain-induced compositional shift. However, strain minimization via a compositional shift competes with a greater rate of relaxation of the InAsP lattice with film thickness at higher substrate temperatures.