Differential reflectance modulation measurements in InGaAs/GaAs and GaAs/AlGaAs multiple quantum wells at room temperature are presented and discussed. In the energy range of the excitonic transitions the spectra obtained are strongly related to the first derivative of the reflectance curves. An exhaustive analysis of the line shape is carried out. It is shown how the modulation signal is related to the fluctuations of the alloy composition and of the well width. An estimate of the degree of inhomogeneity is obtained. The potential of the technique in spectral analysis and for homogeneity characterization is discussed. ©1996 American Institute of Physics.