The geometry and vibrational frequency of XSO and XSO2 (X = F, Cl) radicals in both the ground state and the first excited electronic state, as well as the transition energy between the two states have been studied using the ab initio method embodying Moller-Plesset perturbation theory with correlation energy correction truncated at the second-order (MP2) and fourth-order (MP4), and quadratic configuration interaction, including single and double substitution (QCISD). Both 6-31G* and 6-311G(2d) basis sets are used in the geometry optimization and frequency calculation, and the 6-311G(2df,2pd) basis set in the MP4 and QCISD single-point calculation. The same method is also applied to HSO, producing results in very good agreement with experiments. Molecular orbital calculations suggest that the (A) over tilde(2)A'<--(X) over tilde(2)A '' transition of the XSO radicals involves alternation from the sigma* antibonding orbital into the pi* antibonding orbital, and the (A) over tilde(2)A ''<--(A) over tilde(2)A' transition of the XSO2 radicals experiences the alternation of a nonbonding mixture of p(x), p(y), and p(z) orbitals on the oxygen atoms into a bonding character. The vibrational frequency for S-O stretching of XSO and S-O asymmetric stretching of XSO2 radicals is predicted to be lower in the excited state than in the ground state. The transition energies are best estimated to be 69.0, 55.8, 24.0, and 29.1 kcal mol(-1) at QCISD/6-311 G(2df,2pd)parallel to MP2/6-311G(2d) + Delta ZPE level of theory for FSO, ClSO, FSO2, and ClSO2, respectively, suggesting a low-lying excited electronic state for both FSO and FSO2 radicals.