The desorption of (CO2)-C-13 from the surface of basic oxides based on modified MgO was examined at conditions of chemical equilibrium using a (CO2)-C-13/(CO2)-C-12 isotopic switch method. (CO2)-C-13 desorption rates increase with increasing concentration of (CO2)-C-12 in the gas phase and with increasing carrier gas flow rate, both during isotopic switch and transient temperature-programmed desorption of CO2. These effects are caused by a decrease in the probability of (CO2)-C-13 readsorption and not by faster desorption steps assisted by gas-phase (CO2)-C-12. This isotopic switch method allows the determination of desorption rates and of the density of sites that can bind CO2 reversibly at conditions typical of steady-state catalytic reactions. These measurements require, however, that we include in data analysis protocols any effects of readsorption processes.