S-(-)-perillaldehyde (C10H14O) has been characterized in the gas phase using a Fourier transform microwave spectroscopy experiment in a supersonic molecular beam. Two conformers - with the isopropenyl group in the equatorial position - have been detected and described by a set of molecular parameters including the principal rotational constants and the quartic centrifugal distortion parameters. Quantum chemical calculations indicate that a third conformer might not be observed due to relaxation processes in the jet. The gas phase results are compared with the liquid phase IR-Raman-VCD spectra. Our study shows that gas phase spectroscopy is a powerful tool for characterizing monoterpenes. (C) 2009 Elsevier B. V. All rights reserved.