Densities were measured in the hydrogen chloride solvent system over the normal liquid range of the solvent. In addition to pure hydrogen chloride, solutions of tetramethylammonium chloride, tetramethylammonium bromide, tetramethylammonium iodide, and trimethylsulfonium iodide in hydrogen chloride were measured at compositions up to about 2 M. Attempts to measure densities of tetraethylammonium halide solutions failed, as the salts were found to be insoluble in liquid hydrogen chloride. Solutions were prepared by condensing hydrogen chloride over solute in a graduated borosilicate glass cell. The densities were measured by a direct Archimedean technique using a fused quartz helix to indicate the weight of a submerged sinker. The accuracy of the method is 0.0006 g/cm(3) with a precision of 0.0005 g/cm(3). The measured densities of pure hydrogen chloride are consistent with values reported in other recent studies. The molar volume and the thermal expansion coefficient of the pure liquid are calculated. The molar volume indicates a void space of roughly 50% in the pure liquid; this finding is consistent with the view that liquid hydrogen chloride does not self-associate to any appreciable extent. The thermal expansion coefficient is on the same order of those of other room-temperature molecular liquids but is 1 order of magnitude larger than that of water. The measured densities of the electrolyte solutions are found to obey Root’s equation.