- First published as an Adv vance Article on the web 5th December 2001 We present R2PI and IR±UV double resonance spectra of the nucleobase adenine seeded in a supersonic jet. We show that there is only one tautomer of adenine which absorbs in the wavelength range 36 050 to 36 700 cm À1 . The IR spectra, measured in the range 3200 to 3700 cm À1 , show bands at 3452, 3508 and 3569 cm À1 , which we assign to the symmetric NH 2 , N±H and antisymmetric NH 2 stretching vibrations of a single tautomer of adenine. We compare the experimental IR±UV double resonance spectra with ab initio based normal mode calculations. The observed tautomer is most probably the 9H amino-form of adenine.