Antigorite, the high-pressure variety of serpentine, is a major water carrier in subduction zones. Its equation of state, which controls its stability field at high pressure, was determined at ambient temperature up to 10 GPa by in situ synchrotron X-Ray diffraction in a diamond-anvil cell. No amorphization, phase transition or hysteresis were detected during compression or decompression. Compression is anisotropic, with the c axis twice as compressible as the others. A fit to the second order Birch-Murnaghan equation of state gave V0 = 2926.23(50) ¿3 and K0 = 67.27(123) GPa. In antigorite the atomic assemblage is denser than in lizardite, which may influence phase stability. The antigorite K0 value obtained here is significantly higher than currently assumed in thermodynamic databases. Antigorite P-T stability field is re-evaluated, with major dehydration reaction (and associated potential earthquake) loci occurring at lower pressures, and is found to be consistent with the latest experimental results.