Changes in the crystal structure and grain modifications in titanium oxide (Ti O2) thin films were observed during the photocatalytic oxidation of hydrocarbons. When the hydrocarbon and collodion films were irradiated, single crystalline titanium oxide transformed into polycrystals. The titanium oxide films gradually became network aggregates. These changes were analyzed with a dedicated in situ transmission electron microscope and observed three dimensionally by electron tomography. A detailed analysis of electron energy loss spectra of the samples also revealed that the changes were associated with the loss of oxygen atoms in the Ti O2 crystal lattice. Correlations between the polycrystalline grain size of Ti O2 and its catalyst activity were discussed based on the measured data.