Absorption and fluorescence excitation and emission spectra of the B DNA duplex decamer d[CTGA(2AP)TTCAG]2, where emission from the 2AP (2-aminopurine) base dominates, have been measured as a function of temperature. A low-temperature excitation band in the 260–270-nm region disappears near the duplex melting temperature, Tm = 27 °C, but then reappears at higher temperatures. Singlet-singlet energy transfer thus occurs between the normal DNA bases and the 2AP base in the B-helical conformation and to a lesser extent in the structurally-mobile melted conformation. The measured efficiency of transfer is 4–5% at 4 °C, near 0 at 30 °C, and rises again to 1% at 48 °C. Nearest-neighbor-only singlet transfer is likely. Such transfer does not offer a likely explanation for UV damage distributions in DNA. © 1993, American Chemical Society. All rights reserved.