A neutron study of the crystalline complex of 1,8-bis(dimethylamino)naphthalene (DMAN) with 1,2-dichloromaleic acid (ClMH2) has been carried out at 100 K using the Laue time-of-flight technique. The moieties are planar. The neutron data indicate that both [N—H...N]+ and [O—H...O]− hydrogen bonds in the complex are asymmetric. There are significant differences between the neutron and X-ray temperature factors, C—H, N—H and O—H bond lengths. There is a strong correlation between the neutron and X-ray temperature factors for non-H atoms and no correlation for H-atom temperature factors. According to the neutron data the involvement of a given H atom in a weak C—H...O hydrogen bond can be correlated with the ratio of equivalent temperature factors of the H and non-H atoms to which they are attached.