The volume of periclase (MgO) has been measured by monochromatic X-ray diffraction in a laser-heated diamond anvil cell up to a pressure of 53 GPa and a temperature of 2500 K. The X-ray source was synchrotron radiation at the European Synchrotron Radiation Facility (Grenoble, France). In addition to laser heating, the use of argon as a pressure transmitting medium provided quasi-hydrostatic conditions in the cell assembly. In order to take thermal pressure effect into account, pressure was measured using an internal pressure calibrant (platinum). By analysis of the experimental P-V-T data set the following parameters were obtained: at ambient temperature, K0'=3.94¿0.2 when K0 is fixed to 161 GPa (with a Birch-Murnaghan equation of state); under high temperature, α(P=0,T)=(3.0+0.0012T)¿10-5 K-1; (∂KT/∂T)P=-0.022(3) GPa K-1. The quasi-harmonic Debye model appears to describe correctly the temperature dependence of the volume at high pressure within experimental errors, with the following parameters: &thgr;D0=800 K, &ggr;0=1.45 (Gr¿neisen parameter under ambient conditions), and q=0.8¿0.5. ¿ 2000 American Geophysical Union