Pure rotational transitions of the chlorine monoxide radical have been observed up to v=2 in the X 1 2 Π 3/2 and X 2 2 Π 1/2 states and transitions of the 35 Cl 18 O isotopomer have been observed in natural abundance. Additionally, rotational transitions for levels up to J′=115/2 have been measured in the far infrared. These data have been merged with the existing microwave, submillimeter, and high-resolution infrared transition frequencies and fit simultaneously with a set of isotopically independent parameters. Isotopic substitution of both the Cl and O atoms has enabled the first determination of the electron spin–rotation constant γ=−296.0(43) MHz as well as the Born–Oppenheimer corrections to the rotational constants.