Exptl. evidence is presented for the process of MO Rydbergization postulated by Mulliken, whereby an antibonding orbital continuously changes its character from predominantly Rydberg to predominantly valence on its way to dissocn. By matrix-isolation spectroscopy of the ~A1B1 state of H2O in Ar and Kr matrixes and in ice, the authors show that the absorption threshold behaves like a valence transition while the max. has a behavior typical of Rydberg transitions in condensed matter, in agreement with the predicted changing character of the state. [on SciFinder (R)]