Large errors may occur in the X-ray diffraction determination of epitaxial layer mismatch, thicknesses or/and periodicity if the substrate miscut angle is not taken into account. Misoriented sample rocking curves are indeed strongly affected when the azimuth angle is changed, even in the case of a nearly symmetric reflection. This effect is investigated by directly mapping the reciprocal structure of a GaAs/AlAs multilayer grown on a 4-degrees misoriented GaAs substrate. The measurements, performed using a triple-crystal-like X-ray diffraction technique, agree well with a model in terms of interfaces and lattice plane tilts. Simple relations based on this model are given to deduce the layer structural parameters from the rocking curves.