The S-O stretching fundamental, nu(2), of thionylimide has been recorded at high resolution and rotationally analyzed for the first lime. This strong A-type hand with origin al 1257.5808 cm(-1) is extensively perturbed by the 5(1)6(1) state at 1206.58 cm(-1) through a- and b-axis Coriolis interactions. Even though no nu(5)+nu(6) band transitions could be Found. a complete analysis of the perturbed rotational structure has been achieved. Accurate molecular constants for the "bright" 2(1) state and the "dark" 5(1)6(1) state have been obtained, using a Hamiltonian that includes both types of Coriolis interactions. Further small perturbations at high k(a) and J have been identified as higher-order interactions with the nu(3) band at much lower energy.