In industrialized countries, people spend more than 20 hours a day indoors, being constantly exposed to several pollutants. Among the various pollutant molecules found in closed environments, aldehydes are the most prevalent and, specific attention is given to formaldehyde as a carcinogenic compound and as a ubiquitous substance, because of the large number of sources from which it is released. Photocatalysis was studied here in particular for the reduction of indoor air contaminants; specifically, we investigated the performances of three commercial powdered TiO₂, two nano-sized and one micro-sized, for the photodegradation of formaldehyde alone, and in a gaseous mixture of aldehydes, taking into consideration the most recent studies of indoor pollutants. The first purpose of this study was to prove the effectiveness of TiO₂ on formaldehyde, even when its particles are micro-sized; secondly, the study was directed to the photodegradation reactions of many aldehydes, with specific attention to the fact that they were degraded simultaneously, in a context much closer to reality, but still poorly studied. For this reason photodegradation was monitored by PTR-ToF-MS (Proton Transfer Reaction-Time of Flight-Mass Spectrometer), which is able to detect several molecules without any difficulty or interference.