The crystal structures of carbamoylcholine [2-(carbamoyloxy)-N,N,N-trimethylethanaminium] chloride, bromide and iodide, methoxycarbonylcholine [2-(methoxycarbonyloxy)-N,N,N-trimethylethanaminium] iodide, acetylcholine [2-(acetyloxy)-N,N,N-trimethylethanaminium] chloride and succinylcholine ¿2,2'-[(1,4-dioxo-1,4-butanediyl)bis(oxy)]bis(N,N,N-trimethylet hanaminium)¿ iodide have been redetermined at 105 K in order to obtain detailed and accurate information on the geometry of choline esters and to elucidate the conformationally dependent changes of geometry. The conformational flexibility and the preferred conformations are elucidated based on results obtained from X-ray crystallographic studies and molecular mechanics (MM2) calculations. The usefulness of molecular mechanics calculations for quaternary ammonium ions is discussed.