The gas phase thermodynamic properties of 135 polychlorinated xanthones (PCXTs) are calculated using a combination of quantum mechanical computations performed with the Gaussian 03 program at the B3LYP/6-311G** level. It is found that the chlorine substitution pattern strongly influences the thermodynamic properties of the compounds. The thermodynamic properties of congeners with the same number of chlorines also depend on the chlorine substitution pattern, especially for ortho-substituted congeners. PCXT congeners with one phenyl ring fully chlorinated are found to be the least stable among the analogues. The effect of the chlorine substitution pattern is quantitatively studied by considering the number and position of Cl atom substitution (N-PCS). The results show that the N-PCS model may be used to predict the thermodynamic properties for all 135 PCXT congeners. In addition, the values of molar heat capacities at constant pressure (c(p,m)) from 200 to 1000 K for PCXT congeners are calculated, and the temperature dependence relation of this parameter is obtained using the least-squares method.