We performed studies on the capillary condensation of various substances in optically anisotropic porous silicon layers. Their strong in-plane form birefringence has been utilized to analyze the polarization state of the transmitted light when molecules penetrate into the pores. The polarization state of the transmitted light is correlated with the filling fraction of the pores by an effective-medium model for anisotropic porous materials. The experimentally obtained adsorption/desorption isotherms show hysteresis typical for capillary condensation in porous materials. We discuss the shape of the hysteresis loops in the framework of the morphology of the layers. Pore-size distributions derived from adsorption/desorption isotherms are presented. (C) 2003 American Institute of Physics.