Isoquinolin-3-ol is prepared by reaction of methyl o-formylphenylacetate with liquid ammonia. The 1-methyl, 1-phenyl, and 1,4-diphenyl derivatives are similarly prepared. These compounds exist predominantly as the lactim tautomers in ether solution, whereas in ethanol both tautomeric forms are important. Diels–Alder and photodimerisation reactions of certain isoquinolin-3-ols are described.