P-V-T measurements on magnesite MgCO3 have been carried out at high pressure and high temperature up to 8.6 GPa and 1255 K, using a DIA-type, cubic-anvil apparatus (SAM-85) in conjunction with in situ synchrotron X-ray powder diffraction. Precise volumes are obtained by the use of data collected above 873 K on heating and in the entire cooling cycle to minimize non-hydrostatic stress. From these data, the equation-of-state parameters are derived from various approaches based on the Birch-Murnaghan equation of state and on the relevant thermodynamic relations. With K-0' fixed at 4, we obtain K-0=103(1)GPa, alpha(K-1)=3.15(17)x10(-5) +2.32(28>x10(-8)T, (partial derivative K-T/partial derivative T)(P)=-0.021(2)GPaK(-1), (d alpha/partial derivative P)(T)=-1.81x10(-6)GPa(-1)K(-1) and (partial derivative K-T/partial derivative T)(V)=-0.007(1)GPaK(-1); whereas the third-order Birch-Murnaghan equation of state with K-0' as an adjustable parameter yields the following values: K-0=108(3)GPa, K-0'=2.33(94), alpha(K-1)=3.08(16)x10(-5)+2.05(27)x10(-8) T, (partial derivative K-T/partial derivative T)(P)=-0.017(1)GPaK(-1), (partial derivative alpha/partial derivative P)(T)=-1.41x10(-6)GPa(-1)K(-1) and (partial derivative K-T/partial derivative T)(V)=-0.008(1)GPaK(-1). Within the investigated P-T range, thermal pressure for magnesite increases linearly with temperature and is pressure (or volume) dependent. The present measurements of room-temperature bulk modulus, of its pressure derivative, and of the extrapolated zero-pressure volumes at high temperatures, are in agreement with previous single-crystal study and ultrasonic measurements, whereas (partial derivative K-T/partial derivative T)(P), (partial derivative alpha/partial derivative P)(T) and (partial derivative K-T/partial derivative T)(V) are determined for the first time in this compound. Using this new equation of state, thermodynamic calculations for the reactions (1) magnesite=periclase+CO2, and (2) magnesite+enstatite=forsterite+CO2 are consistent with existing experimental phase equilibrium data.