There are still open questions concerning the initial steps in photocatalytic mechanisms. In this paper, the effects of TiO2 surface modification on the shift of initial mechanism in UV light photodegradation of three model azo dyes, Orange I (OI), Orange II (OII), and Orange G (OG), are reported. Without modification, the direct hole mechanism plays a major role for all the three dyes; the relative percentage of hole mechanism (Ph) is in the order of OI > OII > OG. However, with additives (F- or SO42-), which change the surface chemistry of the semiconductor, the initial steps shift partly from direct hole oxidation to radical induced mechanism; the reaction sites move from the particle surface to the solution. The shift potential is in the order of OI < OII < OG. Adsorption isotherm experiments show that there is no correlation between the adsorption ability of dyes and the photodegradation mechanism. The reaction mechanism is probably related to the dyes' affinity with active species. (c) 2006 Published by Elsevier B.V.