Photoluminescence spectroscopy has been used to study proximity-capped, rapid thermal anneal (RTA) interdiffusion of InGaAs/InP and InGaAsP/InP quantum well structures grown by gas source molecular beam epitaxy (GS-MBE). The interdiffusion is characterized as a function of well thickness and composition, cap layer, as well as RTA time and temperature. It is proposed that the dominant source of intermixing derives from the non-homogeneous nature of the quantum well ternaries and quaternaries, and the associated grown-in defects. These probably occur at the phase boundaries between the InAs-and GaP-rich regions produced during growth due to the existence of a miscibility gap. It is also demonstrated that a thin InGaAs cap layer enhances the thermal stability of such structures by reducing the concentration of defects produced at the sample surface during the anneal.