A low-frequency Maxwell-Wagner relaxation, resulting from space-charge buildup, was observed when BN-SiC composites were subjected to an ac current. This paper discusses the Maxwell-Wagner relaxation and information that it reveals about the microstructure of the composite. A modified Maxwell-Wagner equation for the real part of the permittivity is used to model the data. SiC has a higher dielectric constant than that of BN, The microstructure of these composites is anisotropic, with BN platelets preferentially oriented perpendicular to the hot-pressing direction. Samples measured in this direction exhibit a lower dielectric constant than those measured parallel to the hot-pressing direction. In regard to frequency response, the permittivity of BN shows no dependence on frequency, whereas frequency dependence is seen with increasing SIC content.