We present the first gas-phase Fourier transform infrared (FT-IR) spectra of uncapped (natural) amino acids using a fast thermal heating technique up to around 570 K. The experimental spectra of phenylalanine, proline, valine, leucine and isoleucine in the range 900-3700 cm(-1) show no sign of decomposition and are in good agreement with ab initio calculated frequencies and intensities at the B3LYP/6-31G(d,p) level, if non-hydrogen bonded structures are assumed at the high temperatures of the experiment. (c) 2005 Elsevier B.V. All rights reserved.