The neutral form of the non-coded amino acid sarcosine (N-methylglycine) has been studied in the gas phase using laser-ablation molecular-beam Fourier transform microwave (LA-MB-FTMW) spectroscopy, Two conformers have been unequivocally identified by comparing the experimental values of rotational and quadrupole coupling constants with those predicted by ab initio calculations at the MP2/6-311++G(d,p) level. The most stable conformer is stabilized by an intramolecular hydrogen bond N-(HO)-O-...=C and a cis-COOH arrangement, whereas the second conformer exhibits an intramolecular hydrogen bond (NH)-H-...-O and trans-COOH. The conformational behaviour of sarcosine is discussed in the context of those of glycine and N,N-dimethylglycine. (c) 2007 Elsevier B.V. All rights reserved.