The abs. photodissocn. yield of H2O was measured in Ar matrixes at 160 nm. This energy was chosen as it corresponds to the max. of the photodissocn. yield curve. The abs. quantum efficiency is given by h = NOH/n(160 nm), where NOH is the no. of OH mols. produced following the absorption by H2O mols. of n photons at 160 nm. It is, therefore, necessary to know the abs. photon flux as well as the no. of OH mols. in the sample in order to est. h. [on SciFinder (R)]