In this contribution we analyze the calculation of the quantum efficiency observable in the gas-phase heterogeneous photocatalytic oxidation of toluene. To this end, we carried out porosimety, UV-visible optical, and TEM tomography measurements together with the mathematical modeling of the most significant physico-chemical factors influencing the quantum efficiency observable. Critical factors concern physical properties related to the light-matter interaction as well as the chemical properties defining the outcome of the reaction. Among the firsts, we analyze the effective catalyst surface area illuminated and provide a detailed formulation of the radiation model, including main absorbance, transmittance and reflectance events. For chemical properties not only the activity but also the selectivity was considered. The work was carried out with a dual aim. First, by scanning a basis set containing most common reactor geometries and light sources, we illustrate quantitatively (numerically) and qualitatively (relative among factors) the importance of each one of the above mentioned factors in quantum efficiency calculations. Second, using a series of samples with "continuous" variation of chemical, textural, morphological and optical properties we calibrate the goodness of common simplifications utilized to represent the above mentioned physical/chemical factors and measure their numerical effects in the quantum efficiency observable. Combination of these two approaches provides a general scheme to bundle and interpret all physico-chemical factor effects in quantum efficiency outputs and can serve as a guide to calibrate current quantum efficiency formulations accuracy. (C) 2015 Elsevier Inc. All rights reserved.