We analyze the optical properties of the monoplate compensator under misalignment conditions. The coupling between the boundary conditions and the secular equation in the crystal frame leads to a quartic secular equation, and certain representations of the mode vectors vanish for particular combinations of parameters. Misalignment affects mainly odd harmonics in the transmitted intensity. The even harmonics, which carry sample information, are affected only to second order. Results are in good agreement with experiment. (C) 2003 Elsevier B.V All rights reserved.