A varying refractive index across a wavefront leads to a change in the direction of propagation of the wave(1,2). This provides the basis for phase-contrast imaging of transparent or weakly absorbing materials with highly coherent X-ray beams(3,4). Lattice distortions can also change the direction of propagation of a wave field diffracted from a crystal. Here we report the use of this principle to effect phase-contrast imaging of the domain structure of a ferroelectric material, lithium niobate. A periodically domain-inverted structure for quasi-phase-matching of second-harmonic generation is created in this material, in which the direction of spontaneous polarization is sequentially inverted. Because of complex interactions during domain-inversion processing, this is accompanied by lattice distortions across the domain walls. These distortions split the diffracted wavefront of a beam of coherent X-rays from an advanced synchrotron source, giving rise to a pattern of interference that reflects the underlying pattern of lattice distortions. These results show that this phase-contrast imaging technique with sub-micrometre spatial resolution permits the non-destructive, highly sensitive phase-mapping of various structural defects and distortions introduced into materials during processing.