Pure rotational transitions of the chlorine monoxide radical have been observed up to v=2 in the X(1) (2)Pi(3/2) and X(2) (2)Pi(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 gamma=-296.0(43) MHz as well as the Born-Oppenheimer corrections to the rotational constants. Copyright 2001 Academic Press.