The concentration-dependent self-association of nicotinamide in solution has been studied by 1H and 13C n.m.r. spectroscopy, attendant relaxation time measurements, and osmometric techniques. N-Methylnicotinamide and N, N- dimethylnicotinamide were also studied to evaluate the role of the amide group in the association process. The osmometric studies indicated that the dimethyl -substituted analogue underwent little (if any) self-association, whereas nicotinamide and N- methylnicotinamide did self-associate. The concentration-dependent 1H and 13C chemical shift profiles of nicotinamide and the monosubstituted analogue indicated that the association involved the amide group and did not occur through stacking of the pyridine rings. Spin lattice relaxation (T1) studies indicated that the T1 orthoC)/T1 para C) ratio of nicotinamide decreased with increasing concentration, and that loss of a preferred axis of rotation had occurred due to formation of a large associated species. The T1ratios of either substituted amide analogue were not concentration-dependent. The extent of self-association of the monosubstituted derivative was insufficient to affect the measured T1ratios. These data indicate that the self-association of nicotinamide in aqueous solution occurs primarily through interamide hydrogen bonding.