Photodissocn. of H2O in Xe matrixes was investigated by excitation in the first continuum (~A1B1) by using dispersed synchrotron radiation. The dissocn. threshold near 6.4 eV corresponds to a barrier of .apprx.1.3 eV due to repulsive H-Xe pair interactions and is attributed to a prompt cage exit of the H atoms. The dissocn. efficiency for cryst. samples does not depend on temp. in the 5-40 range. An increase of the dissocn. efficiency with temp. for non-cryst. samples up to the value of cryst. ones is caused by partial annealing. The dependence of dissocn. efficiency on excess kinetic energy is well rationalized in terms of a model based on the mass dependence of energy exchange between the H fragment and the cage atoms that was derived for the case of Ar and Kr matrixes (Schriever, R. et al., 1990). [on SciFinder (R)]