The As/P exchange behavior on a GaAs and an InxGa1-xAs surface exposed to a P-2 flux and on an InyGa1-yP surface exposed to an As-2 flux has been studied by reflection high-energy electron diffraction (RHEED) in a gas-source molecular beam epitaxy system. We found the exposure of InxGa1-xAs to a P-2 flux results in a severe substitution of As atoms by P atoms at the normal growth temperature (520 degrees C), forming a strained transition layer and causing a rough surface within 10 s, which is indicated by a spotty RHEED pattern. The InyGa1-yP surfaces, however, remained relatively stable under an As-2 flux. Two methods (a thin protective-layer and low-temperature growth) for eliminating the As/P exchange have been developed, and both result in abrupt interfaces.