The influence of the microporosity of activated carbons on their CO2 adsorption capacity and on the kinetics of the CO2 adsorption process from ambient environments at low CO2 concentrations has been studied. The experimental results show that the micropore size distribution is more important than the micropore volume or the surface area alone, Thus, activated carbons with low degree of activation, and therefore with a greater number of narrow micropores, have a higher adsorption capacity than those with a higher micropore volume but with wider micropores. The diffusion parameter and the apparent activation energy of the process were also related with the size of micropores.