At, initio molecular orbital (MO) calculations have been carried out for base-hydrogen fluoride (HF) complexes (base = O-3 and SO2) in order to elucidate the structures and energetics of the complexes. The ab initio calculations were performed up to the QCISD(T)/6-311++G(d,p) level of theory. In both complexes, hydrogen-bonded structures where the hydrogen of HF orients toward one of the oxygen atoms of bases were obtained as stable forms. The calculations showed that cis and trans isomers exist in both complexes. All calculations for the SO2-HF complex indicated that the cis form is more stable in energy than the trans form. On the other hand, in Os-HF complexes, the stable structures are changed by the ab initio levels of theory used, and the energies of the cis and trans forms are close to each other. From the most sophisticated calculations (QCISD(T)/6-311++G(d,p)//QCISD/6-311+G(d) level), it was predicted that the complex formation energies for cia SO2-HF, trans SO2-HF, cis O-3-HF, and trans O-3-HF are 6.1, 5.7, 3.4, and 3.6 kcal/mol, respectively, indicating that the binding energy of HF to SO2 is larger than that of O-3. The harmonic vibrational frequencies calculated for cia O-3-HF and cis SO2-HF complexes were in good agreement with the experimental values measured by Andrews et al. Also, the calculated rotation constants for cis SO2-HF agreed with the experiment.