ZnO thin films were obtained by the sol–gel method, using the dip-coating procedure. Glass slides were used as substrates. The sintering temperature ( T s) was varied in the range of 200–600 °C in intervals of 50 °C, in an open atmosphere. Films with 1 and 5 coatings were prepared for each T s. An increase of the grain size from 10 to 34 nm as the T s increased was observed from X-ray diffraction measurements. The thickness of the films prepared starting from five coatings, decreased by 36% when T s increased, and denser films were obtained. This result was corroborated with the refractive index values, calculated from the UV–Vis transmission spectra. The films were tested as a photocatalyst by the photobleaching of methylene blue in an aqueous solution under UV light exposure during 5 h. The photocatalytic activity (PA) increased with T s, around 72% for the films with one coating and 66% for those with five coatings. The samples with one coating and a T s=500 °C showed the best PA. However, the glass substrate had a negative effect on the PA for T s>500 °C, even when the surface morphology of the samples showed an increase in roughness when T s increased. The observed negative effect can be due to the presence of an amorphous compound formed by Si, Zn and O at the glass–ZnO interface.