A set of zwitterionic monomers bearing the 3-ammoniopropanesulfonate moiety is analysed in CDCl3, CD3OD and D2O by one- and two-dimensional H-1 n.m.r. and C-13 n.m.r. spectroscopy. Absolute and relative positions of the n.m.r, signals are determined depending on the solvent used; the refined assignments of the signals in the vicinity of the ammonium group deviate from previously proposed assignments based on standard n.m.r, shift tables. Still, there is no convincing evidence for a special ring current effect due to a six-membered ring conformation of the betaine moiety.