HCl:ImCl ambient-temperature ionic liquids have been investigated through infrared and variable-temperature H-1-NMR spectroscopy as a function of HCl mole fraction (Im+ = 1-ethyl-3-methyl-1H-imidazolium). The infrared spectrum of the dihydrogen trichloride ion, H2Cl3-, has been observed, apparently for the first time. The absence of significant molecular HCI as earlier proposed, is confirmed. Thermodynamic parameters for the reaction H2Cl3-+ Cl- half arrow right over half arrow left 2HCl2-, in which all species are solvated, have been estimated by VT-NMR : DELTAH and DELTAS were determined as -22.8 +/- 1.0 kJ mol-1 and -31.8 +/- 2.9 J K-1 mol-1 respectively. The minimum value of the equilibrium constant for the reaction HCl + Cl- half arrow right over half arrow right HCl2-, is estimated to be on the order of 10(4)-10(5) L mol-1, considerably larger than that in molecular solvents. Im+ ring proton resonances have a marked dependence on the anion fractions of Cl-, HCl2-, and H2Cl3-, with Im-H-2 being the most strongly affected. VT-NMR spectra of Im+ indicate that for X(HCl) < 0.5, all three ring protons are involved in cation-anion interactions while for X(HCl) > 0.5, Im-H-2 is dominant in mediating these. We have interpreted these results in terms of hydrogen-bonded aggregates of anions and cations, the nature of which depends on the ability of the anion population to form hydrogen bonds with Im+. At the extremes we suggest that these aggregates may range from extended structures involving hydrogen bonding to all three ring protons, to ion-pairs involving hydrogen bonding via Im-H-2.