Specific volumes of forsterite and San Carlos olivine have been measured by in-situ X-ray diffraction at simultaneously high pressure and high temperature up to 7 GPa and 1300 K in a cubic-anvil press coupled to synchrotron radiation. No difference could be evidenced between the thermal pressure of the two compounds. It has been shown that the parameter alpha K-T is volume-independent within the volume range investigated in this study for both forsterite and San Carlos olivine. A quasi-harmonic calculation of the high-pressure high-temperature specific volumes of forsterite has been performed and shown to be in good agreement with the experimental results. Then, a self consistent model of all the thermodynamic functions of forsterite was constructed, the input consisting of the experimental ultrasonic and vibrational spectroscopic data exclusively. The model accurately reproduces not only the experimental P-V-T data measured in this study, but also the high-temperature, 1 bar thermal expansion, the adiabatic incompressibility, the constant pressure specific heat and the entropy measurements, without using other a priori information.