TiO2nanoparticles were synthesized by the Sol-Gel method by using 2-propanol as solvent in acid medium (pH1). The samples were annealed at 200 and 500°C and were characterized by BET, XRD-Rietveld refinements, TEM and FTIR. The activity was evaluated by the acetaldehyde photodecomposition in an isolated chamber with an initial concentration of contaminant of 300 ppmv with oxygen (2%) assisted with a 365-nm UV lamp. The test results were compared with those obtained with a commercial catalyst (P25). Improved photoactivity (≍100 % of acetaldehyde in 150 min) was obtained with catalysts annealed at 200°C (TiO2-P-200°C), that showed nanoparticles (≍7 nm) and abundant anatase phase (≍ 63 %) coexist with the brookite phase (≍ 37 %), as well as irregular equiaxial morphology. The samples annealed at 500°C (TiO2-P-500°C), showed an increment in nanoparticles (≍22 nm), different ratio and phase composition (anatase-brookite-rutile), and therefore less activity (≍80 %). This high activity could be explained by the special ratio of anatase-brookite and the dimension of nanometric crystal size. The aforementioned characteristics could be useful in the degradation of reactive organic gases like acetaldehyde either in confined spaces or in the open air.