Competitive 13C and 18O kinetic isotope effects (KIEs) on CO2 reduction catalyzed by Re(bpy)(CO)3Cl (1) under photocatalytic conditions and using Na(Hg) amalgam as a sacrificial reducing agent are reported. The analysis of the measured KIEs rules out the outer-sphere electron transfer mechanism and indicates that CO2 binding to the reduced rhenium complex is the rate determining step in the reduction of CO2 by 1 under these conditions.