Data obtained on rotating-compensator ellipsometers using biplate retarders often exhibit small interference oscillations whose periods are not obviously related to any of the dimensions of the biplate. We show that these artifacts are due to a non-linear second-order interference effect that originates with the boundary condition on the outgoing wave and depends critically on the spacing of the air gap between plates. The latter characteristic makes them component-specific and hence difficult to control in production. They can be eliminated by filling the air gap with an index-matching medium, or by replacing the biplate with a single-plate device. Degree-of-polarization data are in agreement with theoretical predictions. (C) 2004 Elsevier B.V. All rights reserved.