The applicability of utilizing activities instead of concentrations in kinetic expressions has been investigated using the reaction Of CO2 in sodium hydroxide solutions also containing different neutral salts (LiCl, KCI and NaCI) as model system. For hydroxide systems it is known that when the reaction rate constant is based on the use of concentrations in the kinetic expression, this "constant" depends both on the counterion in the solution and the ionic strength which is probably caused by the strong non-ideal behavior of various components in the solution. In this study absorption rate experiments have been carried out in the pseudo-first-order absorption rate regime. The experiments have been interpreted using a new activity based kinetic rate expression instead of the traditional concentration-based rate expression. A series Of CO2 absorption experiments in different NaOH (1, 1.5, 2.0 mol 1(-1))-salt (LiCl, NaCl or KCI)-water mixtures has been carried out, using salt concentrations of 0.5 and 1.5 mol 1- 1 all at a temperature of 298 K. Interpretation of the data additionally required the use of an appropriate equilibrium model (needed for the calculation of the activity coefficients), for which, in this case, the Pitzer model was used. The additions of the salts proved to have a major effect on the observed absorption rate. The experiments were evaluated with the traditional concentration based-approach and the "new" approach utilizing activity coefficients. With the traditional approach, there is a significant influence of the counter-ion and the hydroxide concentration on the reaction rate. The evaluation of the experiments with the "new" approach-i.e. incorporating activity coefficients in the reaction rate expressions-reduced the influence of the counter-ion and the hydroxide concentration on the reaction rate constant considerably. The absolute value of the activity based-reaction rate constant k(OH)(m)- (gamma) for sodium hydroxide solutions containing either LiCl, KCI or NaCI is in the range between 10 000 and 15 000 kg (kmol(-1) s(-1) compared to the traditional approach where the value of the lumped reaction rate constant k(OH)- is between 7000 and 34 000 m(3) (kmol(-1) s-1). Therefore, it can be concluded that the application of the new methodology is thought to be very beneficial especially in processes where "the thermodynamics meet the kinetics". Based on this it is anticipated that the new kinetic approach will first find its major application in the modeling of integrated processes like Reactive Distillation, Reactive Absorption and Reactive Extraction processes where both, thermodynamics and kinetics, are of essential importance and, additionally, activity coefficients deviate substantially from unity. (C) 2007 Elsevier Ltd. All rights reserved.