A perylene-based dye was synthesized and covalently bound to undoped SnO2 nanosized powder. Because of the grafted dye, film sintering is not a straightforward process. In a preliminary step, it is proposed to check the photoactivity of this new powdered material directly by means of the cavity microelectrode technique under green or blue laser illumination through an optical microscope. Taking advantage of the chemical stability of the Sn–C bond, aqueous redox electrolytes were considered. Two types of microcavities were used, a laser-ablated with a residual Pt-coating on the overall cavity inner side, and a chemically etched with the rear Pt contact at the bottom only. Voltammetry and electrochemical impedance spectroscopy (EIS) were used to characterize the response of the SnO2 powder inserted in the cavity. From the results presented here, two contributions have been identified: (i) that of the light insensitive Pt|electrolyte interface due to the Pt-coated inner side when present; (ii) that of the dye-grafted SnO2|electrolyte photoactive interface. It is shown that direct information on available photopotentials for a given redox electrolyte can be readily obtained. From this point of view, a 600 mV open-circuit voltage has been obtained with the new dye-sensitized SnO2 powder in contact with an aqueous bromide-containing electrolyte.