There is a growing interplay between infrared observations of rotation–vibration transitions and microwave measurements of the rotational structure of the vibrational states. This interplay has perhaps been most extensive in HNO3 (e.g., [1], [2] and [3]). ClONO2 is a similar, but more challenging case for both infrared and microwave analysis. In the infrared, the closely spaced transitions make the resolution of individual lines difficult. In the microwave, the large number of thermally populated vibrational states makes assignment of the rotational spectrum of higher lying vibrational states challenging.