This study aimed to demonstrate the behavior of different complexes using IR spectroelectrochemistry (SEC), a technique that combines IR spectroscopy with electrochemistry. Four different Mn and Re catalysts for electrochemical CO reduction were studied in dry acetonitrile. In the case of Mn(apbpy)(CO)Br (apbpy = 4(4-aminophenyl)-2,2'-bipyridine), SEC suggested that a very slow catalytic reduction of CO also occurs in acetonitrile in the absence of proton donors, but at rather negative potentials. In contrast, the corresponding Re(apbpy)(CO)Br clearly demonstrated slow catalytic conversion at the first reduction potential. Switching to saturated CO solutions in a mixture of acetonitrile and 5% water as a proton donor, the SEC of Mn(apbpy)(CO)Br displayed a faster catalytic behavior.