The equilibrium constant Kp and the standard enthalpy, entropy and heat capacity at constant pressure changes for the ClO(g) dimerization are calculated from partition functions. The Cl2O2 partition functions are constructed using recent quantum-chemical data while in the ClO case spectroscopic information is applied. A due attention is given to the Cl2O2 isomerism, viz. to the individual species ClOOCl (C2 symmetry), ClClO2(Cs), and ClOClO(C1). The available observed standard thermodynamic terms are reproduced within their estimated experimental errors, entropy terms yielding a better theory-experiment agreement (comparing to enthalpy case). The theromodynamic terms are evaluated in a wide temperature interval, pointing out some features not amenable from the observations. The data can be useful in concentration evaluations as an intermediate for elucidating ozone-depleting mechanisms.