The optimization of a corrugated wall photo catalytic reactor for air treatment was addressed using the concept of efficiencies in series. A new concept has been introduced: the inner incidence efficiency, associated with the internal configuration of the reactor (for instance, the folding angle in the considered case). The goal of the study was to obtain an optimum for the folding angle, subjected to certain constraints: fixed reactor volume, radiation flux, and gas flow rate. Previous experimentally determined degradation kinetics of a gaseous pollutant was used in the reactor modeling. The simulations of the reactor were performed computationally by using a computational fluid dynamics (CFD) package. The results showed opposed behaviors for the relative catalytic area per unit window area and the relative incident flux with the folding angle. Given that the inner incidence efficiency comprises the product of the two mentioned variables, an optimal folding angle was identified.