A theory for the analysis of experiments involving laser picosecond acoustics with obliquely incident probe light in a two-layer structure is outlined. The reflectance and phase changes of the reflected light are calculated with a theory that takes into account the effects of multiple optical reflections. The sample consists of a single partially transparent layer on a substrate, both with arbitrary optical constants. We discuss the conditions in which one may discriminate between components of the optical modulation of a probe beam arising from the photoelastic effect and from the displacement of the sample interfaces induced by the acoustic strain.