A threshold near 6.8 eV is obsd. for the dissocn. of H2O (and D2O) via the repulsive A(1B1) potential in rare gas matrixes. The threshold varies within 0.2 eV for exchange of Kr with Ar matrix, upon deuteration, and with temp. The observations are consistent with a prompt dissocn. across matrix-induced barriers of about 1.8 eV caused by the repulsive H-rare gas potentials. The increase of dissocn. efficiency above threshold and its variation with matrix and isotope substitution seen in the expts. is rationalized by the mass dependence of energy exchange fragment and matrix and the dependence of the accessible exit cone on kinetic energy. A strong increase in the dissocn. efficiency with temp. is obsd., which is different for Ar and Kr. [on SciFinder (R)]