Contactless electroreflectance measurements at 300 K were performed on two InxGal-xAs/InP ]x = 0.53 (lattice-matched) and 0.75] samples containing three quantum wells (QWs) grown by gas-source molecular beam epitaxy. The spectra consisted of two excitonic transitions (le-l hh and le-l lh), corresponding to the fundamental conduction to heavy (h)- and light(l)- hole transitions, respectively, in the QW portion and a complicated Franz-Keldysh oscillation (FKO) pattern originating in the InP regions. Comparison between the experimental energies of le-l hh/le-llh and a theoretical envelope function calculation (including the effect of strain) made it possible to evaluate the conduction band offset parameters Qc =0.34+0.03 and 0.57+0.03 for x = 0.53 and 0.75, respectively. The InP related FKO beat patterns were analyzed by a Fourier transform method. It was found that the FKO spectra were due to the simultaneous contribution of at least three different fields (106 kV/cm, 36 kV/cm, and 23 kV/cm), which originate in the various interfaces, i.e., substrate/buffer, cap layer/surface, and buffer/QW structure. Identification of the different fields has been accomplished by comparison of the Fourier-transformed spectra before and after sulfur passivation of the structure surface.