The hydrogen atom transfer reaction in indole (NH3)(n) clusters excited at 263 nm to the S-I(pipi*) state is studied in pump-probe experiments with femtosecond laser pulses. For small clusters the reaction is characterized by two successive processes on distinct time scales: the time constant of the primary process is in the sub-ps region, whereas the secondary decay time growing with the cluster size reflects a relaxation process within 25 to 150 ps. A preliminary model of the H-transfer reaction is discussed.