The mechanism of eosin-sensitized photoreduction of benzil with 1-benzyl-1,4-dihy-dronicotinamide—a model compound of NAD(P)H and the behavior of the excited states of eosinhave been investigated.The effect of anthracene as a diffusion-controlled quencher of the photoreac-tion indicates that both excited triplet state and an unquenchable excited singlet state of eosin parti-cipated in the sensitized photoreaction.From the Stern-Volmer plot of quantum yield vs.anthraceneconcentration,the triplet reaction rate constant has been calculated to be 0.78×10~8 L M~(-1)S_(-1) whilethe singlet reaction rate constant determined from quenching of eosin fluorescence by benzil is equalto 7.2×10~9 L M~(-1)S~(-1).The singlet and triplet quantum yields are also determined to be 0.09and 0.18respectively.Since both the singlet and triplet energies of eosin are lower than that of benzil,energytransfer sensitization is not feasible.It is proposed that electron transfer from the excited eosin tobenzil is responsible for the initiation. The mechanism of eosin-sensitized photoreduction of benzil with 1-benzyl-1,4-dihy-dronicotinamide-a model compound of NAD (P) H and the behavior of the excited states of eosinhave been investigated. The effect of anthracene as a diffusion -controlled quencher of the photokinetic typing of the both excited triplet state and an unquenchantly excited singlet state of eosin parti-cipated in the sensitized photoreaction. Flash the Stern-Volmer plot of quantum yield vs. anthracene concentration, the triplet reaction rate constant has been (1) While the singlet reaction rate constant determined from quenching of eosin fluorescence by benzil is equal to 7.2 × 10 ~ 9 LM -1 S -1 ). The singlet and triplet values ​​are also determined to be 0.09 and 0.18respectively. Both both the singlet and triplet energies of eosin are lower than that of benzil, energytransfer sensitization is not feasible. It is proposed that electron transfer from the excited eosin tobenzil i s responsible for the initiation.