The reactivity of Tl(I) species in UV-irradiated TiO2 suspensions is described. It is shown that the reaction pathway undergone by these species depends crucially on the conditions extant in these media. The Tl(I) conversion pathway switches from a reductive one in organic additive-containing suspensions to an oxidative one in the O-2-containing media. Thus the influence of organic or inorganic co-additives, the purged gas (whether Ar or O-2), and platinization of the TiO2 surface, on the extant of initial (dark) adsorption of Tl(I) on the TiO2 surface, and on the initial product of the photoreaction, is described.