4-Nitroanisole photoreacts with n-hexylamine and ethyl glycinate, giving rise to regioselective methoxy and nitro group photosubstitution, respectively. Mechanistic evidence indicates that the latter is produced through a SN23Ar* reaction, whereas the former arises from a radical ion pair via electron transfer from the amine to a 4-nitroanisole triplet excited state. AM1 semiempirical calculations on the actually involved excited states and intermediates indicate that the change of regioselectivity between the ground-state and the triplet-state substitution can be justified on the basis of frontier orbital considerations. On the other hand, neither the frontier orbital nor the net charge can explain the regioselectivity when the reaction involves electron transfer. A discussion of the influence of other previously neglected factors such as the intermediate stabilities is advanced. © 1990, American Chemical Society. All rights reserved.