A key intermediate in the electroconversion of carbon dioxide to carbon monoxide, catalyzed by a manganese tris(carbonyl) complex, is characterized. Different catalytic pathways and their potential reaction mechanisms are investigated using a large range of experimental and computational techniques. Sophisticated spectroscopic methods including UV/Vis absorption and pulsed-EPR techniques (2P-ESEEM and HYSCORE) were combined together with DFT calculations to successfully identify a key intermediate in the catalytic cycle of CO 2 reduction. The results directly show the formation of a metal-carboxylic acid-CO 2 adduct after oxidative addition of CO 2 and H + to a Mn 0 carbonyl dimer, an unexpected intermediate.