The two lowest vibrational states of 35 Cl 35 ClO 2 , v 4 = 1 (A ) and v 6 = 1 (A ), were investigated between 223 and 500 GHz. More than 250 rotational transitions were recorded with J and K a up to 71 and 34, respectively. The spectra are heavily perturbed by strong c-type and weaker a-type Coriolis interactions. Near degeneracies of rotational levels of the two vibrational states having J = 0, K a = 5 to 1, and K a + K c = odd cause moderate to severe perturbations in the rotational structure, preventing the states from being fit as isolated ones. Distortions in the hyperfine structure facilitated the assignment of rotational quantum numbers. Several resonantly interacting levels with K a = 5 to 2 were accessed, and a number of transitions between the states were observed. While resonant Coriolis interaction with K a = 1 occurs only at K a > 40, the effects of this interaction are so severe that nonresonant interaction considerably perturbs the highest K a Q-branches observed. The observed transitions could be fit to within experimental uncertainties employing the first-order Coriolis coupling constants fixed to those from the harmonic force field, sextic distortion constants fixed to those of the ground state, and some higher order Coriolis terms. The energy difference calculated from the fit agrees well with that obtained from the matrix-isolation infrared spectrum. Quadrupole coupling constants were determined for both Cl nuclei and both vibrational states.