Interdiffusion of the group-Ill atoms was interpreted for the structural and optical changes observed in X-ray diffraction and PL spectra of post-growth annealed 1.3-mum Ga1-x InxNyAs1-y/GaAs material systems. Interdiffusion of As and N was observed to be much less than that of group-III in the temperature range studied in this work. A diffusion barrier, GaInNAs, grown on either side of the quantum well reduced interdiffusion very effectively. As a result, a blue shift of photoluminescence caused by annealing was prevented to a large extent, emission intensity was improved, and the spectra were shifted towards longer wavelengths, due to broader effective quantum wells. It was also found that interfacial stress tended to enhance diffusion. (C) 2002 Elsevier Science Ltd. All rights reserved.