We demonstrate a novel method for the growth of abrupt InGaAs(P)/In(GaAs)P heterojunctions by gas source molecular beam epitaxy. We find that exposure of freshly grown InP to an As flux during growth interruption between layers of different compositions results in the substitution of surface P atoms and As atoms, thereby generating a strained transition layer at each interface. By assuring a group‐III stabilized surface during interruption required to grow InGaAs(P)/In(GaAs)P heterointerfaces, As/P substitution can be avoided, thereby resulting in improved interface quality. Heterointerface abruptness was examined by double‐crystal x‐ray diffraction and low temperature photoluminescence. The results show that the interfaces grown with the modified switching sequence are considerably more abrupt than those obtained using conventional sequences where As/P interdiffusion extends over several monolayers. ©1995 American Institute of Physics.