Using Gaussian 03 program, the thermodynamic properties of 135 polybrominated phenoxazines (PBPXs) have been calculated by density functional theory (DFT) at the B3LYP/6-31G* level. Based on the output data of Gaussian, the isodesmic reactions are designed to calculate standard enthalpy of formation (Delta H-f) and standard Gibbs energy of formation (Delta G(f)) of PBPXs congeners. The relations of these thermodynamic parameters with the number and position of Br atom substitution (N-PBS) are discussed, and the order of relative stability of PBPX congeners is theoretically proposed according to the relative magnitude of their Delta G(f). In addition, the values of molar heat capacities at constant pressure (C-p.m) from 200 to 1000 K for PBPX congeners are calculated, and the temperature dependence relations of this parameter is obtained using the least-squares method. Crown Copyright (C) 2009 Published by Elsevier B.V. All rights reserved.