We report on an investigation into the interfacial structure of undoped GaInAs/GaInAsP multiple quantum wells grown by metalorganic chemical vapour deposition (MOCVD), which exhibit a pronounced blue shift in luminescence output upon in-situ thermal annealing at 750°C. Using a recently developed composition mapping technique based on the scanning transmission electron microscope (STEM) in conjunction with energy dispersive X-ray (EDX) analysis, the constituent element concentration profiles across the interdiffused multilayer interfaces are measured with a spatial resolution of less than 2nm and a precision of better than 2–3%. The accuracy of the analysis is significantly improved by employing stoichiometric normalisation factors which compensate for systematic errors due to electron channelling. The results showed that the interdiffusion follows a highly non-linear path due to the relatively fast diffusion of the group V species compared to that of the group III species. This implies an increase in the coherency strain in the multilayer, a result which is supported by five-crystal Xray diffraction analysis of the layers. The samples have also been examined by high resolution electron microscopy (HREM) under chemically sensitive imaging conditions. The analysis of the interfacial chemical profile using HREM must be performed under analysis conditions for which a known and unique relationship between image contrast and composition occurs. This condition may not be satisfied in cases in which more than two chemical constituents interdiffuse and the diffusivities of these elements are not equal, as a range of similar lattice fringe motifs across the interface, representing different “diffusion paths”, could occur. The complementary nature of information provided by HREM and STEM provides a means of resolving this ambiguity.