Optimized geometries, harmonic force fields, and dipole derivative tensors were computed for isoxazole and isothiazole with HF, MP2, DFT, and MCSCF methods using the 6-31G** atomic orbital basis set. The ab initio force fields were scaled to form scaled quantum mechanical (SQM) force fields using the experimental fundamental frequencies for isoxazole-d(0) and -d(3) and isothiazole-d(0), -4-d(1), -5-d(1), and -4,5-d(2). The calculated frequencies confirmed the experimental assignment for isothiazole and its isotopomers and showed up possible misassignments for isoxazole and its -d(3) isotopomer. The computed atomic polar tensors were used to calculate the IR absorption intensities. The best agreement between the calculated optimized geometries and IR absorption intensities and the experimental results was obtained with density functional calculations, but the correlation between the scale factors determined for both molecules was worse than those calculated at the MP2 and HF levels. With MP2 the optimized geometries were slightly worse than those at the DFT level, the calculated IR absorption intensities were in excellent agreement with the experimental IR absorption intensities for the in-plane modes but in poor agreement for the out-of-plane modes, and the correlation between the scale factors determined for both molecules was excellent. The HF- and MCSCF-optimized geometries and IR absorption intensities are slightly worse than the density functional results. Although the geometry and the intensity for the in-plane modes are calculated correctly with MP2, the large disagreement of the out-of-plane modes indicates a strong static correlation. The correlation between the scale factors determined with HF was worse than that at the MP2 level but better than that of the density functional and MCSCF calculations.