We have determined the apparent molar volumes V-phi and apparent molar heat capacities C-p,C-phi of aqueous potassium carbonate and potassium bicarbonate solutions in the ranges (0.014 less than or equal to m/(mol (.) kg(-1)) less than or equal to 0.51) and (278.15 less than or equal to T/K less than or equal to 393.15) at the pressure p = 0.35 MPa. Corrections for speciation due to hydrolysis and disproportionation in solution were applied using Young's rule, and semi-empirical equations representing (V-phi, m, T) and (C-p,C-phi, m, T) for the species {2K(+), CO32-(aq)} and {K+, HCO3- (aq)} were fitted to the experimental results. We have used these equations to estimate the change in volume Delta(r)V(m), change in heat capacity Delta(r)C(p,m), enthalpy change Delta(r)H(m), entropy change Delta(r)S(m), and equilibrium molality quotient pQ for the second proton dissociation reaction from aqueous carbonic acid. (C) 2004 Elsevier Ltd. All rights reserved.