The energy efficiency of the photocatalytic conversion of gas-phase organic pollutants was studied using a redesigned and scaled up Photo-CREC-Air reactor. This photocatalytic unit has the unique feature of allowing an accurate analysis of the irradiation field by establishing macroscopic balances and in-situ measurements. The Photo-CREC-Air reactor operates in a batch mode with the photocatalyst supported by a stainless steel mesh being irradiated by 8 UV lamps. Kinetic modelling, Quantum Yields (QY) and Photochemical Thermodynamic Efficiency Factors (PTEF) were calculated using data for acetone , acetaldehyde and isopropanol photocatalytic degradation in ambient air utilizing a Degussa P25 photocatalyst. It is shown that the Photo-CREC-Air reactor is suitable for the determination of kinetic and adsorption parameters, given design with excellent irradiation usage and fluid-catalyst contact. In this respect, quantum yields for acetone, acetaldehyde and isopropanol supersede the value of 1 (equivalent to 100%) with PTEFs remaining in both cases below the level of 1 as required by thermodynamics