Densities of aqueous solutions of HCL at molalities from O.1 mol . kg(-1), to 6.0 mol . kg(-1), temperatures from 298.15K to 623 K, and pressures of 10 MPa and 28 MPa have been measured with a vibrating-tube flow densimeter. Heat capacities of the above solutions at temperatures from 302 K to 623 K and a pressure of 28 MPa were measured using a flow heat-capacity calorimeter. Standard-slate values for the volumes and heat capacities have been obtained from the apparent molar properties by extrapolation to infinite dilution using the Fitter ion-interaction model. Our experimental results were combined with available literature results on volumes and heat capacities to derive appropriate equations for the temperature and pressure dependencies of the Fitter model. The volumetric and heal capacity results for HCL(aq) are in good agreement with published values al low temperatures. Al higher temperatures, the qualitative behavior of volumes and heat capacities for HCL (aq) shows clearly the transition from a fully dissociated strong electrolyte to an almost completely ion-paired weak electrolyte. Standard-state values calculated from this work are consistent with the literature values for the volume and heal capacity change on the ionization of water.