Base-catalyzed efficient hydroxylation of isoquinoline-1,3(2 H,4 H)-diones with air under transition-metal-free and reductant-free conditions was established. This methodology is essentially mild and compatible with a broad range of substrates, including aryl, heteroaryl, and alkyl groups. Also, the product could be simply transformed into a hydroxylated tetrahydroisoquinoline core structure through a reductive process.