Calorimetric, phase equilibrium and in situ equation of state data for antigorite, talc, and dense hydrous magnesium silicate (DHMS) phases have been obtained to construct a self-consistent thermodynamic data set that ensures internal consistency, well-behaved extrapolation and physically reasonable values of ΔH¿, S¿, V¿, K, K' and α for talc, antigorite, and hydrous phase A. Used in conjunction with temperature profiles of subducting oceanic lithosphere, the calculated phase equilibria show that there is no barrier to subducting substantial amounts of water to depth of 400--600 km in colder slabs, since the slab can remain in the stability field of hydrous phases throughout its descent. Once water is brought to these depths, it can probably diffuse into hotter mantle regions via hydrous β-Mg2SiO4 or by other mechanisms, indicating that the transition zone is unlikely to be dry. ¿ 1998 American Geophysical Union