An approach is presented, in which such fundamental molecular parameters as geometric structural parameters and potential ones are used to analyze observed spectra of a molecule with an internal rotor. As an example, geometric structural parameters, harmonic vibrational force constants, barrier heights to the internal rotation, and barrier derivatives with respect to molecular internal coordinates are determined by fitting them to observed torsion-rotational spectra for methanol and its isotopomers. By using these determined parameters, all the unreduced torsion-rotational molecular constants up to fourth order are calculated numerically. (C) 2002 Elsevier Science B.V. All rights reserved.