Base-catalyzed reversal of a psoralen-DNA cross-link has been observed under denaturing alkaline conditions at elevated temperatures. the cross-link was formed between 4/-(hydroxymethyl)-4,5',8-tri-methylpsoralen and the two thymidine residues (T) on opposite strands of the double-stranded DNA formed from the self-complementary oligonucleotide 5/-GGGTACCC-3/. In contrast to the photoreversal of the cross-link, which yields mostly the furan-side monoadducted oligonucleotide [Cimino, G. D., Shi, Y., & Hearst, J. E. (1986) Biochemistry 25, 3013–3020], base-catalyzed reversal of the cross-link yields only pyrone-side monoadducted oligonucleotides as identified on the basis of their mobilities on a 20% poly-acrylamide-7 M urea gel and their chemical and photochemical properties. A mechanism has been proposed to explain the base-catalyzed reversal reaction. This observation suggests a way to make pyrone-side monoadducted DNA. It also suggests that caution must be taken when psoralen-adducted DNA is treated under denaturing alkaline conditions.