Abstract The structures of the cations derived from the simplest and most common mono- and bicyclic delta lactams, i.e. N-methylpiperidin-2-one and quinolizid-4-one, and simple acids are discussed on the basis of spectroscopic and X-ray structural data. In addition to these tertiary lactam salts, the structure of the simplest secondary delta lactam salt, piperidin-2-one hydrochloride, is also discussed and compared with caprolactam hydrochloride.