Apparent molar volumes Vφ and apparent molar heat capacitiesCp, φ were determined for solutions of aqueous imidazole and of aqueous imidazolium chloride at molalities m= 0.015 mol · kg−1 to 0.5 mol ·kg −1, at temperatures T= 278.15 K to 393.15 K, and at the pressure p= 0.35 MPa. Values of Vφ were obtained from density measurements using a vibrating-tube densimeter. Values of Cp, φ were obtained from heat capacity measurements using a twin fixed-cell, power-compensation, differential-output, temperature-scanning calorimeter. We fit the results by regression to empirical equations that describe the ( Vφ, m,T ) and ( Cp, φ, m, T) surfaces for each of the two systems. We then used these regression equations to estimate the volume and heat capacity changes (ΔrVm , m, T) and (ΔrCp, m , m, T) for the proton dissociation reaction of aqueous imidizolium ion. Our results illustrate the unique thermodynamic properties of protonated imidazole and of neutral imidazole in aqueous solution. Numerical integration of our (ΔrCp, m , m, T) surface, using literature values for the enthalpy change ΔrHm and the molality concentration equilibrium reaction quotient pQa, were performed to obtain the surfaces (ΔrHm , m, T), (pQa , m, T), and entropy change (ΔrSm , m, T) for the proton dissociation reaction of imidazolium ion.