When a dissociation equilibrium is present, the complete phase space should be allocated consistently to either the associated or dissociated species without duplication or omission of any important region. While this problem is trivial in many systems at low pressure, it becomes important and complex for an ionizing system at high pressure. A feasible division is proposed and equations are presented for both the dissociated (ionized) and associated species. It is suggested that any extensive tabulation of properties of ionizable species be made in a manner consistent with the principles outlined.