Nowadays, nanostructured semiconductor materials offer promising opportunities for a new generation of materials such as TiO2 nanoparticles with improved properties for their application in the environmental catalysis field. It is well known that the phocatalytic activity of the TiO2 nanoparticles is strongly dependent on the surface area, crystal size, phase composition and synthesis method. Thus, the preparation conditions clearly affect the photocatalytic activity of the TiO2 nanoparticles. This work deals with the study of the structure of TiO2 nanoparticles that were synthesized by the sol-gel method (using isopropanol as solvent), and calcined at 200 and 500°C. The obtained samples were characterized by the XRD-Rietveld refinement, BET and TEM techniques; and tested in the photodecomposition of acetaldehyde. The evaluations were carried out at room temperature by using CH3CHO (300 ppmv), O2 (2.0 %) in helium balance in a quartz glass photoreactor (gas phase) with a 365-UV light lamp. According to the results, the sample that presented the highest activity in the photocatalytic oxidation of acetaldehyde (96.4%) was the one annealed at 200 °C. This sample showed the following proportion of phases: anatase (62.88%) with a tetragonal structure (a=0.3790926, b=0.3790926, c=0.9495732) nm; and b) brookite (37.12%) with an orthorhombic structure (a=0.9167624, b=0.5416461, c=0.5210546) nm. The surface area was 189 m2/g and the average crystal size was 7.03 nm. From the results, it can be seen that this material showed high activity in the photocatalytic degradation of acetaldehyde because of: the presence of a mixture of the anatase (higher proportion) and brookite phases, nanometric crystal size and high surface area obtained in this TiO2 material. According to the aforementioned, this material can be considered as a good option for the decomposition of acetaldehyde and other volatile organic compounds (VOCs) in confined spaces.