Development of an x-ray-transparent capsule that is suitable for water- saturated experiments has allowed the high-pressure and high-temperature synthesis of 10 angstrom phase from talc + H2O to be observed in situ in a multianvil apparatus with synchrotron radiation. The reaction talc + H2O = 10 angstrom phase was observed within 20 minutes at 6 gigapascals and 500°C. The higher pressure vapor-absent reaction talc = 10 angstrom phase + enstatite + coesite was also observed. The 10 angstrom phase is therefore a good candidate for transporting water into Earth's mantle at subduction zones. The in situ technique may allow the hydration of other high-pressure mantle phases to be observed under dynamic conditions.