The fluorescence decay of J-aggregates adsorbed on AgBr microcrystals was measured with picosecond resolution between room temperature and 5 K. Progressive photobleaching of the J-aggregate fluorescence at temperature >40 K indicated that excitation of the J-aggregates led to an irreversible chemical reaction. It was found that meaningful measurements could be obtained at this irreversible system only with the careful control of exposure to light. Rate constants for electron transfer quenching were determined from fluorescence decay curves, assuming that electron transfer was the primary reaction step. A fit was made of the temperature dependence of the rate constant to the Levich-Jortner and Huang-Rhys electron transfer models. Many acceptor states with different energies in the conduction band of AgBr were considered. The standard free energy difference was found to be about 50 cm(-1) uphill, and the electron transfer integral was found in the range 5-10 cm(-1). An upper boundary of 300 cm(-1) was determined for the reorganization energy in the J-aggregate system, in contrast to about 3000 cm(-1) reorganization energy recently determined for a related monomer system. The small reorganization energy suggests that delocalized states in the J-aggregates are involved in the electron transfer reaction, and the system behaves like a solid state system. This can explain the known high efficiency for spectral sensitization in photographic systems at temperatures down to 100 K.