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, K'₀ = 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; (∂K_T/∂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: θ_D0 = 800 K, γ₀ = 1.45 (Grüneisen parameter under ambient conditions), and q = 0.8 ± 0.5.