The energetic requirements for electron transfer to the conduction band of silver bromide from a series of cyanine dyes have been studied using both picosecond time-resolved and steady state fluorescence techniques. The selected dyes exhibit monomeric absorption spectra in the adsorbed state and have excited state donor energies that are calculated to span a range about the conduction band edge of AgBr. Rate constants and yields for these electron transfers were extracted from the picosecond fluorescence lifetime measurements. Yields were also obtained through independent measurements of the steady state fluorescence of these dyes and were correlated with estimates of the free energy of the reaction to depict a threshold for electron transfer with respect to the conduction band of AgBr. The threshold compares well with model curves from electron transfer theory and with literature reports of photographic relative quantum yield measurements on similar monomeric systems at silver halide surfaces. This correlation supports the electron transfer model for spectral sensitization of the silver halides. Contrasts are drawn with the sharp energetic thresholds expected for aggregated dyes.