We report a study of the vacuum ultraviolet (VUV) surface photochemistry of H2O (D2O) films on a graphite substrate at 80 K. Experiments utilized a He discharge lamp to generate VUV photons. For 21 eV excitation, H+ (D+) is the only ionic fragment observed in desorption. When 41-eV photons are used, H+ (D+) is again observed: however, photochemically produced H3O+ (D3O+) is also detected. The ratio of D+ to D3O+ depends upon the water coverage. Furthermore, the D3O+ signal is larger than D+, whereas the H3O+ signal is smaller than H+ for irradiation of adsorbed D2O and H2O, respectively. At low coverages where the average coordination of water is also lower, D3O+ production is enhanced compared with higher coverages. The formation of D3O+ is attributed to the reaction of the photodissociation product D+ with adsorbed D2O. We suggest that the opening of the D3O+ product channel at 41 eV and its preponderance over D+ is related to the higher kinetic energy of, and consequently greater momentum transfer by, the D+ ions created through D2O photodissociation at this photon energy. We also suggest that there is a propensity for a hydrogen bonded O-D to break preferentially over a free O-D bond after photoexcitation.