We have investigated the effects of interdiffusion and its technological parameters on the subband structure in compressively strained InGaAsP quantum wells (QWs) using photoreflectance and photoluminescence techniques. p-i-n laser structures with three QWs were grown by gas source molecular beam epitaxy and capped with dielectric films deposited by electron cyclotron resonance plasma enhanced chemical vapor deposition and annealed using a rapid thermal annealing process. A numerical real-time wave-packet propagation method including static electric field, strain in the wells and barriers, and error function interface diffusion modeling is used to calculate the transition energies for the diffused QWs. It has been shown that the shift of the energy levels due to the interdiffusion related changes of the well confinement potential profile is a consequence of two competing processes: a change of the well width and an effective increase of the band gap energy resulting in a net blueshift of all optical transitions. Moreover, it has been found that quantum well intermixing does not significantly influence the built-in electric fields distribution.