C-13, C-14, and O-18 kinetic isotope effects (KIEs) in the reduction of CO2 on a Mg surface were estimated employing the Bigeleisen formalism within the framework of conventional transition-state theory. Linear and three bent four-atom configurations were considered for the transition state of the isotope-fractionation governing step of the reaction mechanism. We investigated several different decomposition modes of the transition states and the influence of diagonal force constant variations and geometry of the presumed transition state on the calculated KIEs. A comparison of the calculated results with the experimental data indicated two possible structures of the transition state. The terminal oxygen atom and the magnesium atom are in trans and cis positions, respectively, regarding the central C-O bond. The reaction coordinate motion is described by a symmetric stretching vibration in the terminal C-O and Mg-O bonds and an asymmetric stretching vibration in the central C-O bond.