Electrocatalytic CO, reduction was studied by using a modified graphite electrode coated with cobalt octabutoxyphthalocyanine (CoPc(BuO),) and dipped in an aqueous electrolyte. The CoPc(BuO), worked as a catalyst to produce CO with higher activity and selectivity than the non-substituted CoPc. Under typical conditions at pH 4.4, the most active and selective CO, reduction was achieved at -1.30 V with turnover number of the catalyst _ 1.1 X lo6 h-' and the selectivity of the produced CO/H, N 4.2. The high activity was ascribed to the electron-donating BuO substituents of the complex, which would facilitate the coordination of CO, as well as the electron transfer from the complex to the coordinated CO, molecule. The use of a poly(4-vinylpyridine) membrane to disperse the complex decreased the activity. The mechanism was investigated by the use of in situ potential-step chronoamperospectroscopy (PSCAS) in a homogeneous CoPc(BuO),/pyridine solution. In the absence of CO,, distinct two-steps visible spectral changes were observed following the first and second reductions of the complex. However, under CO, atmosphere, only the one-electron reduced species was present under steady state. It was proposed that the third reduction of a CO,-coordinated CoPc(BuO), should take place for CO production.