Electron spin resonance (ESR) and diffuse reflectance spectroscopy (DRS) were applied to study the origin of the strongly enhanced photographic sensitivity of red-sensitized (dye RS) AgCl systems upon addition of a Ultraviolet- (UV-) absorbing supersensitizer (SS). Both techniques were used to identify the radicals formed in this system upon spectral (red) as well as intrinsic (UV) excitation. Upon coadsorption of SS on the AgCl/RS system, the intensity of the photoinduced ESR signal is strongly increased. The ESR signals detected in systems with and without SS are very similar in line shape, line width, and g value, suggesting the formation of RS radicals. This was confirmed by the observation of a pair of absorption bands in the DRS spectrum, demonstrating that the radicals are formed on the RS molecules for all systems and ruling out supersensitization by hole trapping on the SS molecules for these systems. The effect of the supersensitizer is shown to be an overall increase in electron transfer efficiency from the dye molecules RS to the AgCl crystals. Indirect evidence is presented for the partitioning model, in which the electron transfer is facilitated by a rise of the lowest unoccupied molecular orbital level of the dye molecules, resulting from a reduction of the effective size of the RS aggregates.