Methyl tert-butyl ether (MTBE) is the basic oxygenated motor fuel additive in Europe and is included in volatile organic compounds (VOCs), which can produce photochemical oxidants. In the present study the gas-phase photocatalytic oxidation (PCO) of MTBE over illuminated titanium dioxide was carried out at ambient temperature in a plug flow reactor. The intermediates detected are mainly tert-butyl formate and acetone, while the final products are CO2 and water. The system was sensitive to the oxygen concentration, for concentrations up to 15% (v/v). Moisture had a positive effect on the reaction, obtaining an optimum value near 45% relative humidity for 200 ppmv MTBE initial concentration. A reaction scheme has been proposed for the interpretation of the experimental results and a kinetic study was conducted, using the Langmuir-Hinshelwood kinetics equation. The MTBE rate constant was 1.545 x 10(-6) M s(-1) g(cat)(-1) for the reaction without moisture and 2.46 x 10(-6) M s(-1) g(cat)(-1) for the reaction in the presence of moisture and the adsorption constant was 2.187 x 10(5) M-1 independent of humidity. (c) 2008 Elsevier B.V. All rights reserved.