Grazing incident X-ray reflection topography was used to evaluate the crystal perfection of ZnSe epilayers grown by metalorganic vapour phase epitaxy on low-dislocation-density (1 0 0) GaAs substrates. Grazing incident topographs were recorded from 224 reflections at a wavelength and an incident angle of 0.1334 nm and 0. 17∘, respectively. We found that there is a remarkable difference between samples with ZnSe epilayer thinner than the critical thickness and thicker samples; lattice imperfections in thin ZnSe heteroepitaxial layers consist of small changes of the ZnSe lattice constant and bending of the crystal planes.