The effect of zinc diffusion on InGaAsP/InP single and multiple quantum well structures was studied by secondary ion mass spectrometry (sims), Auger electron spectroscopy (AES), capacitance‐voltage measurements, photoluminescence, and x‐ray diffraction. Significant interdiffusion of In and Ga is found. The structures are stale against annealing without the presence of zinc. Interdiffusion of As and P is negligible. In a multiple quantum well Zn diffusion at 500 °C causes In and Ga intermixing: The AES profiles have become flat. At higher diffusion temperatures an ordering is found such that now the Ga concentration is at a maximum in the original InP layers. This can be explained by minimization of the free energy, which is balanced by an increasse in mismatch strain energy. Photoluminescence shows the interdiffusion starts at temperatures above 420 °C. By the Zn diffusion the lattice parameter values of the InGaAsP and InP layers are changed and the average value is decreased, as was shown by x‐ray diffraction on a multiple quantum well structure. This is substantiated by results of x‐ray rocking curves.