The photocatalytic degradation of formic acid in suspended and immobilized systems, with and without oxygen addition, are compared. In the immobilized system, oxygen addition to the reactor appeared to increase the efficiency, not only because oxygen acts as an efficient electron scavenger, but also due to increased transfer in this two-phase reactor. This immobilized system had an efficiency comparable to that of the suspended system. The addition of oxy :en to the immobilized system appeared to increase the quantum yield with a factor 4, whereas the addition of oxygen to the suspended system hardly had any effect.