To develop new photoinduced electron-transfer (PET) reagents, we established a method for predicting the fluorescence quantum yields (phi) of the benzofurazan compounds bearing an aliphatic substituent group having an n-electron. The PET process occurred sufficiently to reduce the phi values in the benzofurazan compounds bearing an aliphatic moiety, which had a high quenching ability. The quenching ability was estimated by the molecular orbital calculation and Stern-Volmer plotting. The phi values of the benzofurazan compounds could be controlled by changing the quenching ability of a substituent group. We succeeded in designing a PET reagent for peroxyacetic acid (PAA), 4-ethylthioacetylamino-7-phenylsulfonyl-2,1,3-benzoxadiazole (EPB), using the established method for predicting the phi values. EPB and its oxidized derivative were separated by reversed-phase HPLC and fluorometrically detected at 479 nm with excitation at 362 nm. The attained detection limit for PAA was 105 fmol (S/N = 3) and the cross-reactivity toward hydrogen peroxide was very low, indicating EPB is a highly sensitive and selective reagent for PAA.