Degradation of formic acid was investigated using a novel photoelectrocatalytic reactor with a porous titanium plate as an air distributor as well as an electrode. By the comparison of the COD removal efficiencies from the solution containing formic acid for three processes, direct electro-oxidation, photocatalysis and photoelectrocatalysis, a significant photoelectrochemical synergetic effect was observed in the photoelectrocatalysis process. The synergetic effect was assessed with a new parameter, synergetic factor (SF). The effects of various operating conditions, such as applied cell voltage, treating time, airflow, initial pH value, catalyst amounts and conductivity of solution, on the SF were investigated as well. It was found that the SF was considerably dependent on these operating conditions, moreover, not simply proportional to them.