Polarized transient bleaching signals of J-aggregates of 1,1′-diethyl-2,2′-cyanine or pseudoisocyanine on colloidal silica were obtained by using picosecond pump-probe spectroscopy. The laser was tuned to the peak of the J-band at 570 nm. This absorption band arises from the exciton states of the aggregate. The nonexponential signals were proportional to the laser intensity. However, the decay kinetics appeared to be independent of the laser intensity. For 0 < t < 160 ps, the absorption anisotropy was constant and equal to 0.42 ± 0.04. These results suggest that singlet-singlet exciton annihilation is not occurring and that the excitons are delocalized.