This work evaluates the effect of the catalyst coated surface per reactor volume, illumination mechanism (back side (BSI) or front side (FSI) illumination) and light source on heterogeneous TiO2 photocatalysis using a mili-photoreactor, for the oxidation of a volatile organic compound in a gas stream. A thin film of TiO2-P25 was uniformly deposited on the front borosilicate window of the mili-photoreactor (BSI mechanism) or on the network of channels and chambers imprinted in the back stainless steel slab (FSI mechanism) using a spray system. BSI and FSI mechanisms were assessed using two different UVA LED systems (9 LEDs -LED9 and 18 LEDs -LED18), UVA lamps and simulated solar light as radiation sources. Under BSI and solar light the maximum reaction rate (r(dec)) obtained was 1.0 mu mol min(-1), whereas a 1.7-fold increase was attained using BSI-LED9. Although under FSI mechanism the catalyst coated surface per reactor volume is three times higher than under BSI, the maximum r(dec) for FSI showed only a 1.4 and 1.1-fold increase using simulated solar light and LED9, respectively, when compared with BSI. In order to provide a better light distribution over the catalyst coated surface, a new LED plate with 18 LEDs was designed and tested. Under FSI or BSI mechanisms and using the LED18 system, a similar to 3.6 fold increase on the maximum r(dec) was observed when compared to the LED9 system. However, even for the LED18 system, similar maximum reaction rates are observed for BSI and FSI mechanisms. This means, that regardless of the light source configuration used, similar to 60% of the catalyst coated surface is not illuminated.