Postcombustion CO, capture using calcium looping (CaL) is a rapidly developing technology based on the reaction between CaO and CO2. The influence of the presence of steam in the flue gas on the carbonation reaction has been barely studied in the literature. Experiments using a thermogravimetric analyzer (TGA) were carried out to study the effect of steam over the very short reaction time where the carbonation reaction takes place under the kinetic-controlled regime. A simple reaction particle model is applied to interpret the results obtained and to derive reaction rate constants and support the quantitative discussion on steam effects. Results obtained in this work indicate that steam has no influence on the reaction rate constant.