Thallium(I) is shown to be a sensitive probe of titania particle/aqueous solution interfaces in the dark and under ultraviolet (UV) illumination. We show that experimental conditions can be chosen such that photocatalytic reduction or oxidation of this probe is avoided. Thus, any changes in the measured solution levels of this probe can be simply attributed to corresponding variations in its adsorption tendency on the TiO2 particle surface. Data are presented on how this interfacial probe tracks modulations in the oxide surface charge induced either by variations in solution pH or by UV irradiation.