The absorption spectrum of H2O in Ne, Ar, Kr, Xe and ice matrixes in the region of the 1st ~A 1B1 continuum is reported. Substantial blue gas-to-matrix shifts are obsd. for the max., which increase with decreasing matrix cage size. The absorption thresholds show much weaker shifts. These features are interpreted in terms of Rydbergization of the (3s/4a1) MO correlated to the ~A state. The photochem. is strongly affected by the environmental effects on the excited state potential surface. An energy threshold to photodissocn. is confirmed, consistent with a potential barrier due to the cage and a prompt exit of the H fragment. The photodissocn. quantum yield shows a linear dependence with excess kinetic energy in Ar and Xe matrixes, in agreement with recent MD simulations also favoring prompt exits as the dominant mechanism to permanent dissocn. [on SciFinder (R)]