|The focus of this work is on the homogeneous catalytic CO2 reduction. A manganese/iron catalyst were investigated. Both catalystsshow activity in regard to the photocatalytic/electrocatalytic CO2 reduction. With in situ IR-spectroscopy the photocatalysis of the manganese catalyst was studied. Both catalysts were investigated with IR-spectroelectrochemistry. Mechanistic studies of the reduction process were feasible. With quantum chemical computations a description of the mechanism was possible. CO was proven to be the product of the CO2-reduction in both cases. Intermediates were identified.