The structure of l-ornithine hydrochloride, C5H13N2O Cl-, has been redetermined at 100 K by single-crystal X-ray diffraction within a project that aims to generate accurate bond-distance restraints for the invariom refinement of proteins. The high-resolution data were subject to an invariom and a multipole refinement, and the resulting electron densities on a grid were compared. Improvements in the conventional R factor obtained by multipole modelling were smaller than in other structures containing solely the elements CHNO owing to Cl core scattering. Cruickshanks diffraction-component precision index and Stevens & Coppens suitability factor are discussed. © International Union of Crystallography 2007.