We report a cross investigation of the effect of interdiffusion on the photoluminescence and Raman spectra of a single quantum well of InGaAs (80 Å wide) sandwiched between two large InGaAsP barriers. First, we investigate the blue shift of the recombination line (2 K) after annealing at 650 and 750°C from 15 min to 2 h. We assume one single diffusivity coefficient for all atomic species (i.e., conservation of the lattice matching after annealing) and deduce the amount of intermixing through a model calculation. We find average diffusivity coefficients D=9.5×10 -3 Å2 s-1 and D=2×10-1 Å2 s-1 at 650 and 750°C, respectively. This agrees well with previous measurements reported for the parent system InGaAs/InP and supports an activation energy EA=2.54 eV. Next we investigate, on the same series of samples, the change in phonon frequency associated with the GaAs longitudinal-optical-like mode in the active InGaAs layer. To connect quantitatively the change in wave number with the change in arsenic composition, versus annealing sequence, one must use a carefully checked calibration curve. We show that the linear relationship Δy/Δω=4×10 -2(cm) provides a satisfactory agreement with the results of our photoluminescence investigation. This gives for the amount of arsenic leaving the well Δy≊-5% per h1/2 at 650°C and -20% per h 1/2 at 750°C, respectively.