Apparent molar volumes V-phi and apparent molar heat capacities C-p,(phi) 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 V-phi were obtained from density measurements using a vibrating-tube densimeter. Values of C-p,(phi) 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 (V-phi, m, T) and (C-p,(phi), m, T) surfaces for each of the two systems. We then used these regression equations to estimate the volume and beat capacity changes (Delta(r)V(m), m, T) and (Delta(r)C(p),(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 (Delta(r)C(p),(m), m, T) surface, using literature values for the enthalpy change Delta(r)H(m) and the molality concentration equilibrium reaction quotient pQ(a), were performed to obtain the surfaces (Delta(r)H(m), m, T), (pQ(a), m, T), and entropy change (Delta(r)S(m), m, T) for the proton dissociation reaction of imidazolium ion.