Molecular dynamics simulations of monoclinic (P2(1)/b) hydroxy- and chlorapatite were undertaken in the range 498 K < T < 1298 K, and for pressures up to 7.5 GPa. The all-atom Born-Huggins-Mayer force field, that had been previously used to successfully describe the room temperature isotherms of both compounds, was also used in this work. The isothermal sets of p-V data generated by simulation were each fitted to the three-parameter form of the isothermal Parsafar and Mason equation of state (EoS) with an accuracy better than 0.07%. Taking the temperature dependence of the coefficients into account, it was found that the MD data are satisfactorily reproduced by the universal EoS. The isothermal compressibility coefficient dependence with pressure can be described by a linear relation. (C) 2007 Elsevier B.V. All rights reserved.