The liquidus region of the HCl:ImCl:AlCl3 ionic liquid system has been determined through manometric measurement of HCl solubility as a function of pressure for 0 < P-HCl < 1 atm at 295 +/- 1 K. Analysis of the pressure-composition isotherms thus obtained, in conjunction with spectroscopic data, reveals that dissolution of HCl in these systems is associated with reactions of the general form HCl(g) + X(-)(1) reversible arrow HCIX(-)(1), where X(-) drop Cl-, HCl2-, H2Cl3- AlCl4-, or Al2Cl7-. In the binary HCl:ImCl system, the HCl-derived proton exists as HCl2- H2Cl3-, and H3Cl4-; the equilibrium constants for the reaction above are 715 +/- 76, 3.28 +/- 0.05, and 0.74 +/- 0.02 atm(-1) for X(-) drop Cl-, HCl2-, and H2Cl3- respectively. In Lewis acidic HCl:ImCl:AlCl3 melts (X(AlCl3), > X(ImCl)), HCl is complexed as the species AlCl4 . HCl- and Al2Cl7 . HCl-. The corresponding equilibrium constants were determined as 0.135 +/- 0.001 and 0.109 +/- 0.001 atm(-1) for X(-) = AlCl4- and Al2Cl7-, respectively. In Lewis basic HCl:ImCl : AlCl3 systems, all species with the exception of Al2Cl7 . HCl- contribute, though the equilibrium constants associated with the reaction above are less well-defined, being dependent on the specific composition of the melt. Liquid phase equilibria such as reported earlier from this laboratory are obtained by combining two or more of the dissolution equilibria above such that molecular HCl does not appear in the result. Electrochemical profiles of HCl dissolved in HCl:ImCl:AlCl3 ionic liquids are reported as a function of HCl pressure for 0 < P-HCl < 1 atm at 295 +/- 1 K. In Lewis basic systems, the electroactivity of HCl2-, H2Cl3-, and AlCl4 . HCl- is observed, while in Lewis acidic systems, two electroactive protic species are present : AlCl4 . HCl- and Al2Cl7 . HCl-. The reduction potential of Al2Cl7 . HCl is ca. 0.7 to 1.0 V more positive than that of other protic species, confirming that the proton of Al2Cl7 . HCl- is exceptionally labile. Based on this observation, we conclude that Al2Cl7 . HCl- is the entity responsible for Bronsted superacidity observed in the Lewis acidic melts. It is demonstrated that the reproducibility of voltammograins requires strict control of the HCl pressure.