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An analytical electron microscope was used for direct measurement of the concentration profiles of In1-xGaxAsyP1- y quantum wells (QWs) and barriers grown by molecular beam epitaxy on GaAs substrates. The well and barrier layers had compositional differences on the group III (In/Ga) sublattice only, the group V (As/P) sublattice only, and on both sublattices. These were annealed over a range of temperatures (700-950degC), and the resultant changes in the QW widths and compositional profiles were determined along with the changes in the photoluminescence (PL) emission wavelength. The structures were annealed either uncapped or capped with either a 100-nm-thick layer of low temperature (250degC) grown InGaP (LT-InGaP) or with CVD-grown SiO2. The LT-InGaP layer contains excess phosphorus expected to be present as P-antisite defects. This was used to enhance interdiffusion on the group V sublattice during annealing, producing a blue-shift in PL response. The SiO2 capping leads to outdiffusion of Ga from the top GaAs layer producing additional group III defects that enhance interdiffusion of the group III sublattice. The interdiffusion activation energies and diffusivities were obtained from Arrhenius plots for each of groups III and V QWs profile changes. The compositional profiles of the QW after annealing are used to infer the defects involved in the interdiffusion process on each sublattice.