A dynamical study has been performed to determine front first principles the molecular volume Of C6H6 and C6D6 in the gas phase, starting from a normal-mode analysis and using anharmonic potential energy profiles at DFT level to determine vibrational eigenfunctions for all 30 degrees of freedom and using a Monte Carlo procedure to determine the appreciable ranges of variation of the coordinates of all atoms. The gas phase study reveals that the intrinsic volume Of C6D6 is always the smallest, even though it increases faster with temperature than that Of C6H6. In Order to explain the experimentally observed fact that C6D6 Volume is the largest at hight temperatures, the likely effect on molecular motions caused by the crystalline environment of a given molecule has to be considered.