The photochemilcal reactivity of benzalaniline apd its derivatives has been studied. Photoirradiation of a benzalaniline derivative .in tetrahydrofuran (THE) afforded a 1 : 1 THF adduct. The effective excitation band was the absorption around 260 nm named band II giving the singlet second photoexcited state. In the series, the reactivity highly depended upon the electron donating-withdrawing property as well as the position of the substituent. The reactive derivatives have been classified into three categories ; the derivative with an electron donating group at the 4-position, with an electron withdrawing group at the 4'-position, and with a halogen at the 4-position. The individual key-steps for the three categories, namely, hydrogen abstraction at the singlet second photoexcited state, successive electron and proton transfer at the singlet second photoexcited state, and hydrogen abstraction at the triplet second photoexcited state, have been proposed, respectively, on the basis of a mechanistic study using the electrolyte effect, observation of transient intermediates, quenching experiments, and the deuterium isotope effect. The characteristic positional dependence of the substituent effect on the reactivity has been discussed using semi-empirical MO calculation.