3-Hydroxy-, 3-hydroxy-1-methyl-, and 3-hydroxy-6,7-dimethoxy-1-methylisoquinoline have been shown by ultraviolet spectroscopy to exist as lactim tautomers in non-hydroxylic solvents such as diethyl ether, and as lactam tautomers in water. 1-Chloro-3-hydroxyisoquinoline is in the hydroxy- or lactim form in all common solvents, and 3-hydroxycinnoline is in the lactam form in all solvents tried.