We describe a novel approach to determining absorption coefficients in thin films using luminescence. The technique avoids many of the difficulties typically encountered in measurements of thin samples, Fabry–Perot effects, for example, and can be applied to a variety of materials. We examine the absorption edge for GaAs/AlGaAs multiple quantum well structures with quantum well widths ranging from 54 to 193 A˚. Urbach parameters and excitonic linewidths are tabulated.