A red wing in the absorption of H2O in the 1st continuum which extends beyond 200 nm was induced by the matrix. A threshold energy of 6.95 eV was found for the quantum efficiency of permanent dissocn. of H2O into OH and H. This corresponds to an addnl. barrier of 1.8 eV due to the matrix cage. The barrier was attributed to the sum of repulsive H-Ar pair potentials in an interstitial site of D3 symmetry, which must be surmounted by the H atom on the way from the initial substitutional H2O site to the final stable Oh site of the abstracted H atom. [on SciFinder (R)]