The electrochemically-assisted photocatalytic degradation of 4-chlorophenol (4-CP) using immobilized TiO2 particulate films has been investigated by analyzing reaction intermediates under a variety of experimental conditions. The degradations were carried out in both nitrogen- and oxygen-saturated solutions to explore the role of reduced oxygen species and molecular oxygen in the formation of reaction intermediates and in the mineralization of 4-CP. The degradation rate can be greatly improved even in a nitrogen-saturated atmosphere by applying an anodic bias to the TiO2 film electrodes. 4-Chlorocatechol (4-CC) is the predominant intermediate observed in oxygen-saturated solutions, whereas hydroquinone (HQ) is the primary intermediate in nitrogen-saturated solutions. Molecular oxygen plays an important role in the enhancement of the electrochemically assisted photocatalytic decay rate of 4-CP and the subsequent degradation of reaction intermediates, viz., 4-CC and HQ.