Kinetics for CO2 absorption into 5-30 wt-% sodium carbonate solutions and 5-50 wt-% potassium carbonate solutions upto 70 degrees C were studied in a string of discs apparatus under conditions, in which the reaction of CO2 could be assumed pseudo-first-order. The experimental data were evaluated based on the use of activities in the reaction rate expressions. The second order kinetic constant for the CO2 reaction CO2+OH- <-> HCO3- at infinite dilution is discussed and an expression for it is obtained up to 70 degrees C. The difference between the activity and concentration based kinetic constants were found to be small at low concentrations, where the apparent Henry's law constant is close to that at infinite dilution in water. However, at high concentrations (high apparent Henry's law constants), the difference was bigger. Using the activity based approach, the second order kinetic constant was calculated, compared to the second order kinetic constant in infinite dilution and found to be independent of both carbonate concentration and the cation present in the solution. (C) 2010 Elsevier Ltd. All rights reserved.