The use of quaternary carbon relaxation data, including the effects of deuteration thereon, to obtain information about chemical structures is considered. It is shown that a simple qualitative analysis of the effects of deuteration is subject to a number of potential errors. Structural inferences should be based only on a rigorous quantitative analysis of the data. The effects of deuterium substitution on the relaxation behaviour of benzamide in solution have been measured. The data are accounted for satisfactorily by treating the molecule as an axially symmetric ellipsoid. Furthermore, it is found that deuteration has no significant effect on the rotational diffusion of benzamide in solution.