Monte Carlo computer simulation has been used to interpret and model observed single-crystal diffuse X-ray scattering data for pentachloronitrobenzene, C(6)Cl(5)NO(2). Each site in the crystal contains a molecule in one of six different basic orientations with equal probability. However, no short-range order amongst these different orientations has been detected. The strong, detailed and very distinctive diffraction patterns can be accounted for almost entirely on the assumption of random occupancy of each molecular site, but with very large local relaxation displacements that tend to increase the neighbouring distances for contacts involving NO(2)...NO(2) and NO(2)...Cl with a corresponding reduction for those involving Cl...Cl. The results show that the mean NO(2)...NO(2) distance is increased by approximately 0.6 Angstrom, compared with that given by the average structure determination.