Standard thermodynamic function changes (Delta G(theta)(0), Delta H-theta(0), Delta S-theta(0) and Delta C-p theta(0)) are reported for proton dissociation of acridinium cation in aqueous solution. These values have been determined from a series of precise and accurate acid equilibrium constants, K,. The equilibrium constants have been obtained spectrophotometrically over a range of temperatures from 2 to 66 degrees C at several ionic strengths (I = 0.005, 0.007, 0.01, 0.05 and 0.25 M). Although the ionic strength does not affect in general the Gibbs free-energy values, the results suggest a possible influence on entropy and enthalpy changes.