The effect of TiO₂ surface fluorination on the hydrogen peroxide evolution occurring in photocatalytic runs was investigated employing the azo dye Acid Red 1 (AR1) and two model organic molecules with acidic properties, i.e. formic acid (FA) and benzoic acid (BA), as substrates of oxidative degradation. While AR1 and BA photocatalytic degradation on fluorinated titanium dioxide (F–TiO₂) was markedly faster than on unmodified TiO₂, because of enhanced hydroxyl radical formation, H₂O₂ concentration during the photodegradation of both substrates on F–TiO₂ was lower, possibly because of the reduced rate of interfacial electron transfer. By contrast, FA underwent slower photocatalytic degradation on F–TiO₂, but, at the same time, hydrogen peroxide concentration was relatively high, while no H₂O₂ could be detected during FA photodegradation on unmodified TiO₂. Photocatalytic runs in the presence of the nitrate anion, able to react with the CO₂˙⁻ species produced from FA oxidation, but not with conduction band electrons, demonstrated that CO₂˙⁻ plays a relevant role in H₂O₂ formation during FA degradation on F–TiO₂. In fact, surface fluoride, having a shielding effect at the semiconductor–water interface, not only inhibits the photocatalytic decomposition of H₂O₂, but also favours CO₂˙⁻ desorption and reaction with dissolved O₂, generating H₂O₂. By contrast, CO₂˙⁻ mainly gives electron transfer to the conduction band of naked TiO₂ and surface reduction of the photocatalytically produced H₂O₂.