When solutions of 2-hydroxyphenazine (1), 3-hydroxyacridine (2), and 7-hydroxyquinoline (3) were irradiated with UV light at 77 K, they became deeply colored. The color of the solutions reverted to the original one, when they were left in the dark at 77 K or at a higher temperature. We ascribe the photochromism to proton tautomerization, which is most likely induced by an intermolecular proton transfer in the excited state.