Parameters to correct protonation constants of carboxylate ions, defined as concentration quotients, for ionic strength dependence have been calculated from measured values of the constants obtained at 298 K with NaCl solutions at ionic strengths between (0 and 5.6) mol.kg(-1) and with KCl solutions at ionic strengths between (0 and 4.5) mol.kg(-1). Mean values of specific ion interaction theory (SIT) and Pitzer interaction parameters for carboxylate anions and for some synthetic and natural polycarboxylates have been derived. They show a strong correlation with the charge on the carboxylate ion, similar to that previously found for protonation constants of carboxylate ions in solutions with (C(2)H(5))(4)NI and (CH(3))(4)NCl as a background electrolyte. Any individual interaction parameter, x, can be calculated from the simple relationship: x = (x) over barz(2), where z is the charge on the carboxylate anion and (x) over bar is a global average scaled to unit charge. Both SIT and Pitzer interaction parameters give good fits to experimental data. The strong correlation between the SIT and the Pitzer parameters, proposed in previous papers, is confirmed. The temperature dependence of SIT interaction parameters was obtained from protonation enthalpy values with NaCl solutions, and empirical relationships are proposed for the temperature dependence.