The incorporation of group V materials in GaInAsP grown on InP substrates by gas-source molecular beam epitaxy was studied. The study concentrated on the alloys with energy gaps corresponding to wavelengths near 1.1 μm. An abrupt change was earlier observed in the alloy composition of GaInAsP (λ≈1.05−1.3μm) as a function of substrate temperature. In this paper, I will present new results concerning the abrupt change. Reflection high-energy diffraction was used to examine the changes in the surface reconstruction of the growing layers in order to illuminate the origin of the abrupt change.