Laser flash photolysis studies at 355-nm on the photoreactions of the benzophenone (BP) and N,N-dialkyl-1-naphthylamine, DANA (N,N-dimethyl-1-naphthylamine, DMNA, and N,N-diethyl-1-naphthylamine, DENA) system have been carried out with and without H2O and methanol in acetonitrile (ACN) at 295 K. In the nanosecond time scale, triplet energy transfer from triplet BP ((BP)-B-3*) to DANA occurs with the efficiency phi(TET) (0.74 for DMNA and 0.61 for DENA) regardless of the presence of H2O and methanol. After the formation of triplet DANA ((3)DANA*), me triplet exciplex (3)(DANA...BP)* with weak charge-transfer character is produced with the equilibrium constant K-1 (10 M(-1) for DMNA and 9 M(-1) for DENA) between (3)DANA* and BP. The mechanism for the formation of (3)(DANA...BP)* is shown in Scheme 1. In the presence of H2O and methanol, it is found that the intraexciplex electron transfer takes place to give the BP anion (BP.-) and DANA cation (DANA(.+)) radicals in the hydrogen-bonded triplet exciplex (3)(DANA...BP)(HB)* by H2O or methanol. The mechanism for the production of DANA(.+) and BP.- is proposed in Scheme 2. The equilibrium constants K-2 for the formation of (3)(DANA...BP)(HB)* with H2O and methanol obtained are 0.55 and 0.45 M(-1) for DMNA, 0.50 and 0.40 M(-1) for DENA. The rate constants k(et) for the intraexciplex electron transfer induced by hydrogen bonding are determined to be 2.5 x 10(7) s(-1) for DMNA and 1.4 x 10(7) s(-1) for DENA. It was revealed that the driving force for intraexciplex electron transfer is the negatively enlarged reduction potential of BP in (3)(DANA...BP)(HB)* due to the hydrogen bonding to the carbonyl group of BP in (3)(DANA...BP)*.