In the present study, the photocatalytic degradation of methylene blue (MB), using TiO2-Ag synthesized by sol-gel method, as a catalyst under UV light, was studied. The samples synthesized were characterized by SEM, EDAX, FTIR, XRD and TEM. The average size of the particles was 10 nm. Tests were carried out in a batch reactor, magnetically stirred, with external irradiation using UVB lamp of Hg - 18 W, having intensity of incident radiation 2.1 W/m2. The effects of some parameters, such as catalyst dosage, pH and initial acetone concentration were investigated. Results showed that the optimal catalyst concentration was 0.03 g/L, at concentration of MB solution of 1.25∙10-5 mol/L. The mechanism of degradation of MB in presence of acetone was proposed. The results obtained in this work are very promising, using TiO2-Ag as photocatalyst, especially in presence of acetone as photosensitizer, when degradation percentage increases from 60% for TiO2 without Ag to 92.38% for TiO2-Ag and 0.1% acetone and 97.25% for 0.2% acetone. The photocatalysis followed pseudo-first order reaction kinetic behaviour, with the rate constants ranging from 0.0009 to 0.11 min-1 under the studied conditions. The reaction rate depends nonlinear of acetone concentration, the constants of polynomial function being calculated.