In this paper the applicability of using activities instead of concentrations in kinetic expressions has been investigated using the reaction of CO2 with various hydroxide solutions as a model system. For this system 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 counter ion in the solution and the ionic strength. Using the data of Pohorecki, it could be revealed that the use of activities reduces the influence of the ionic strength on the derived rate constant to a very large extent. Hence the kinetic constant using the activity based approach seems much more like a real constant than the "constant" using the traditional kinetic expression using concentrations. Besides this, the absolute values of the rate constants for the three different counter-ions also moved closely together. This points out that applying one fixed rate constant, independent of the counter-ion or the ionic strength, might be sufficient for describing the reaction rate in the studied system. A large advantage of using activities in kinetic expressions is that the fundamental description of kinetics and thermodynamics is consistent which each other so that at equilibrium the net reaction rate (based on the kinetic expression) is indeed zero.