Torsion-rotational parameters have been determined for C-12 and C-13-methanol isotopomers CH3OH with O-16, 17, and 18 using a recent formulation [Duan and Takagi, Phys. Lett. A 207, 203 (1995)] of the centrifugal distortion effects from potential parameters for a molecule containing a threefold symmetric internal rotor. The calculated parameters, especially the constants representing interactions between torsion and rotation, are used to interpret the relationships among the terms in reduced Hamiltonian and in the analysis of the observed torsion-rotational spectrum. Molecular parameters are calculated from potential energy surfaces for methanol to check the quality of these surfaces. The calculated parameters are compared with parameters obtained from global fits to large experimental data sets. The good agreement between the calculated centrifugal distortion terms and those derived from fits to spectra demonstrates that the derived formulas provide a useful tool for understanding the physical origins and mass dependence of the molecular parameters.