The peculiarities of high-resolution measurements in grazing-incidence diffraction (GID) are studied, both theoretically and experimentally. It is shown that complete discrimination between coherent reflection and diffuse scattering due to defects in GID requires a three-dimensional mapping of reciprocal space. These measurements can be performed using a combination of analyzer crystal and position-sensitive detector for angular analysis of scattered x-rays in mutually perpendicular planes. The equations for the resolution function of GID experiments are given and applied to the interpretation of GID measurements taken from an AlAs/GaAs superlattice. The discrimination of diffuse scattering due to interfacial roughness in the superlattice is demonstrated. ©1997 American Institute of Physics.