At certain wavelengths, depending on the thickness of a thin transparent dielectric layer on an absorbing substrate, a standing‐wave condition can be produced that efficiently couples theSwave to the substrate, simulating a cavity resonance in the dielectric and maximizing the complex reflectance ratio. Under these conditions high sensitivity is obtained to the nature of the interface between the dielectric and the substrate. We illustrate the method by applying it to plasma‐oxidized GaAs samples using scanning ellipsometry from 1.5 to 5.6 eV. As‐grown samples exhibit a composite transition layer at the interface consisting of a mixture of oxide, unoxidized GaAs, and elemental As. A crystalline As layer is formed at the interface by annealing in N2at 550 °C.