We report room temperature surface photovoltage (SPV) spectroscopy studies of both as-grown and interdiffused InAs quantum dash (QD)–in–InAlGaAs quantum well (QW) structures grown by molecular beam epitaxy on (100) InP substrates. The interdiffusion is achieved by means of rapid thermal annealing at 800 ºC for 30 s. The SPV spectra reveal step-like structures related to the optical absorption in the QDs, QWs, InGaAlAs separate carrier confinement layer, InAlAs lower cladding layer and InP substrate. The annealing results in a high-energy shift of these spectral structures. The shift is attributed to the group-III atoms intermixing from the thermal induced interdiffusion across the heterointerfaces. The effect of interdiffusion is larger for the elements with larger surface-to-volume ratio (QDs and QWs). The blue shift of the QD transition is observed also in the photoluminescence spectra. The results contribute to the optimization of the technological procedure for QD bandgap tuning via the interdiffusion technique. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)