A normal coordinate analysis was carried out for the CH 3 radical to calculate the inertia defects and the centrifugal distortion constants. The calculated values were in fair agreement with the experimental results on the ν 2 band recently obtained by infrared diode laser spectroscopy. Third-order anharmonic potential constants were estimated to calculate vibration-rotation interaction constants and the equilibrium CH bond length. The conventional vibration-rotation interaction theory was extended to assess the effects of the large negative anharmonicity of the ν 2 mode on molecular parameters obtained from the observed spectra. The v 2 dependence of the C rotational constant was discussed by this treatment.