The photocatalytic transformation of phenol has been investigated on naked TiO2 and on TiO2/F (0.01 M F-) at pH 3.6 in the presence of different alcohols (tert-butyl alcohol, 2-propanol, and furfuryl. alcohol). On the basis of a detailed kinetic analysis and the time evolution of the intermediates, it is suggested that on naked TiO2 the oxidation of phenol proceeds for 90% through the reaction with surficial bound hydroxyl radical, the remaining 10% via a direct interaction with the holes. On TiO2/F the reaction proceeds almost entirely via homogeneous hydroxyl radicals because of the unavailability of surface-bound hydroxyl in the presence of fluoride ions. The use of alcohols as a diagnostic tool for the analysis of the photocatalytic mechanism is discussed.