In this paper periodically domain-inverted (PDI) ferroelectric crystals are studied using high-resolution X-ray diffraction. Rocking curves and reciprocal-space maps of the principal symmetric Bragg reflections in LiNbO3 (LN) (Λ = 5μm), KTiOPO4 (KTP) (Λ = 9μm) and KTiOAsO4 (KTA) (Λ = 39μm) are presented. For all the samples strong satellite reflections were observed as a consequence of the PDI structure. Analysis of the satellites showed that they were caused by a combination of coherent and incoherent scattering between the adjacent domains. Whilst the satellites contained phase information regarding the structure of the domain wall, this information could not be rigorously extracted without a priori knowledge of the twinning mechanism. Analysis of the profiles reveals strain distributions of Δd/d = 1.6 × 10-4 and 2.0 × 10-4 perpendicular to domain walls in KTP and LN samples, respectively, and lateral correlation lengths of 63μm (KTP), 194μm (KTA) and 10μm (LN). The decay of crystal truncation rods in LN and KTP was found to support the occurrence of surface corrugations
