A method has been developed for measuring optical absorption in epitaxial semiconductors in which the layer structure is grown on a substrate of narrower band gap and opposite conductivity‐type so that the intensity of light transmitted can be monitored by measuring the photovoltage generated at the buried junction using a transparent surface contact. This arrangement provides a quick method of locating the wavelengths of features in the absorption spectrum. However, it is also possible to obtain an absolute measurement of the transmitted fraction using a control sample from which the absorbing layer structure has been removed by etching, usually a matter of etching only a few microns. This result is independent of surface reflections and the linearity and spectral response of the junction. The method is described with particular reference to GaAs/AlGaAs multiple quantum well structures, though it is also applicable to single homogeneous layers, and the experimental results are in good agreement with those obtained by conventional transmission measurements on samples from which the substrate has been removed.