Chlorite crystallising in hydrated mantle peridotite is a potential source of water for subduction zone volcanism. The reaction clinochlore+enstatite=forsterite+pyrope+H2O is the important reaction for defining the stability of chlorite in these rocks. It has been investigated in phase-equilibrium experiments in piston-cylinder and multi-anvil apparatus between 2 and 5 GPa. The reaction has a steep, negative P-T slope at low pressures, with brackets at 860-880 °C at 2.5 GPa, 840-860 °C at 3.0 GPa, and 820-840 °C at 3.5 GPa. The reaction flattens out with increasing pressure to 5.0 GPa at 650 °C. Chlorite compositions close to the reaction were determined from unit-cell parameters measured using high-resolution synchrotron powder diffraction. These showed a decrease in Al content with increasing pressure along the reaction, consistent with calculations using a previously published thermodynamic data set. The experimental results are consistent with the results of earlier studies on the reaction defining the maximum thermal stability of clinochlore, clinochlore=forsterite+pyrope+spinel+H2O. They have been used in the derivation of revised thermodynamic data for clinochlore in the latest version of the previously published data set. The new results show that chlorite in subducting slab or overlying mantle wedge could dehydrate below subduction zone volcanoes and be a major source of the water required for melting. However, the dehydration reaction is not sufficiently pressure-dependent to be responsible for the narrow range in depth from the volcanoes to the slab, and chlorite will not be the only hydrous mineral involved.