Neutron diffraction with isotope substitution of nitrogen, chlorine and hydrogen nuclei is used to investigate the hydration region of both ions and the possibility of ion pairing in aqueous solutions of tetramethylammonium chloride (TMACl) at 0·5m, 1·0mand approximately 2·0mconcentration. The use of a maximum-entropy method to calculate the radial distribution functions is discussed. The H-H correlation function for the water in the 1·0msolution is measured and compared with that for pure water. The nitrogen-substitution results are also compared with a hard-sphere model calculation. The results show that the TMA hydration is apolar rather than ionic, and is consistent with a disordered water-cage structure, while the water in the hydration sphere does not appear to be more ordered than in bulk water. There is no significant change in the hydration structure measured at the different concentrations, although at the highest concentration there may be evidence of the close approach of the anion and cation.