The 10-Å phase (TAP) is a hydrous magnesium silicate that forms from the reaction of talc with H2O at high pressures. Its high-pressure, low-temperature stability means that it could be a storage site for H2O in subduction zones. We have determined the position of the TAP dehydration reaction, TAP = enstatite + coesite + H2O, in phase-equilibrium experiments from 5.0 to 7.1 GPa. Because previous studies had suggested that the composition of TAP is a function of synthesis duration, we used a TAP sample that was synthesised for 392 h. Over the pressure interval of our experiments, the dehydration reaction is isothermal, occurring at a temperature of ~690°C. It is coincident, within experimental uncertainty, with the position of the dehydration reaction of TAP synthesised in short experiments (up to 46 h). Above 7.5 GPa, TAP breaks down to enstatite+stishovite +H2O. This reaction has a negative dP/dT and terminates at an invariant point involving the 3.65-Å phase at ~9.5 GPa,500°C. The zero volume change implied by the isothermal reaction TAP=enstatite+coesite+H2O was used to calculate the interlayer H2O content of TAP along the reaction. A best-fit H2O content of 1 H2O pfu was obtained. This H2O content is independent of TAP synthesis conditions, suggesting that variations in previously measured H2O contents of TAP occur during quenching and decompression of the samples. The stability of TAP in the Earth is probably limited to cold subduction zones, but in these, it could persist to 300 km depth.