We have investigated the adsorption of thymine on Au(111) with in-situ infrared spectroscopy. Using IR spectroscopy we have been able to probe the structure and bonding of the ＇chemisorbed＇ thymine phase. IR bands in the 1800-1550 cm(-1) region, which have a major contribution from stretching vibrations of the carbonyl groups of thymine, exhibit large spectral shifts between thymine in aqueous solution and the chemisorbed phase. By contrast, ＇physisorbed＇ thymine shows no IR bands in the spectral region studied, which is consistent with a flat-lying adsorbate. In-situ IR spectroscopy provides compelling evidence for the orientational change of adsorbed thymine occurring concomitantly with the most pronounced anodic current peak in the voltammogram. We have compared the in-situ IR spectra for chemisorbed thymine with IR spectra of metal co-ordination complexes of thymine and uracil. These comparisons provide evidence that chemisorbed thymine bonds to the Au(111) surface through both carbonyl functionalities and a deprotonated N3.