The problem of ambiguity in the torsion–rotational Hamiltonian is considered as applied to a CH3–XH molecule having a torsion-type oscillation. The parameters of ambiguity are determined on the basis of analysis made by O.N. Ulenikov [J. Mol. Spectrosc. 119, 144–152 (1986)]; the reduction of the effective Hamiltonian is presented.