The photolysis of the hydronium cation H3O+ has been studied at the extreme ultraviolet wavelengths of 35.56±0.24 nm (34.87±0.24 eV) and 21.85±0.17 nm (56.74±0.44 eV) using a crossed ion-photon beam setup at the free-electron laser FLASH. Coincidence photoelectron and photofragment spectroscopy was performed at 21.85 nm, where both inner and outer valence ionization are allowed, and revealed that the XUV photolysis of H3O+ is by far dominated by ionization of outer valence electrons forming the 1A1 and 2E states of the dication H3O2+. The dications were found to dissociate into the channels H2O++H+ (72±4%), OH0+2H+ (18±6%), and OH++H++H0 (10±1%). A kinematic analysis of the H2O++H+ channel after photoabsorption at 35.56 nm (where only outer valence ionization is possible) showed dissociation into excited states of the water radical ion, where the 1A1 state breaks up into the linear Ã 2A1 state of H2O+ and the 2E state decays into the strongly bent B̃ 2B2 state. Finally, from the 2E state of H3O2+, dissociation into OH0(X 2Π)+2H+ was identified to occur with a near linear dissociation geometry.