UV-laser irradn. (hn = 6.4 eV and 5.0 eV) of the water bilayer adsorbed on a Pd(111) surface leads to mol. desorption and to conversion of the adsorbed state as manifested in thermal desorption spectra. The latter effect is attributed to photodissocn. of water on the surface. Time-of-flight measurements show that water mols. desorb with a translational energy of .apprx.600 K for both photon energies indicating a nonthermal process. While desorption is largely suppressed with adsorbed multilayers, conversion within the first layer still proceeds. The dependence of the desorption yield on angle of incidence and polarization of the light reveals substrate excitations as the dominant primary step. A strong variation of cross sections with isotopic substitution is obsd. This is interpreted as evidence for the operation of a mechanism involving excitation onto an isotope-independent excited potential energy surface followed by rapid deexcitation to the ground state so that, of the total no. of species excited, only a small mass dependent fraction actually fragments or desorbs.