The photofragmentation of H2O has been studied by fluorescence spectroscopy at photon energies between Ehν=16.9–54.5 eV. The primary photon beam was monochromatized undulator radiation supplied from the UVSOR synchrotron radiation facility. The fluorescence in the wavelength range of 280–720 nm was dispersed with an imaging spectrograph. The dispersed spectra exhibit the hydrogen Balmer lines of H*[n2LJ′′→2 2LJ″″(n=3–9)] and the emission band systems of H2O+[Ã 2A1(0,v2′,0)→X̃ 2B1(0,0,0)], OH+(Ã 3ΠΩ,v′→X̃ 3Σ−,v″), and OH(Ã 2Σ+,v′→X̃ 2ΠΩ,v″). The fluorescence cross sections for these transitions have characteristic dependences on Ehν and vibrational quantum numbers. The cross section summed over the Balmer lines takes a minimum value at Ehν=21.7 eV and is very small even at 24.9 eV beyond which it steadily increases with increasing Ehν. This behavior is understood as that the superexcited states correlating with H*(n⩾3)+OH(Ã 2Σ+) are too repulsive to be accessible below Ehν∼30 eV by the Franck–Condon transitions from H2O(X̃ 1A1) and as that the Balmer emission below 30 eV is mainly due to the H*(n⩾3)+H(n=1)+O(3Pg) channel. The appearance energy of the OH+(Ã 3ΠΩ,v′→X̃ 3Σ−,v″) transitions is found to be ca. 25.5±0.3 eV. This value is much higher than the dissociation limit of 21.5 eV for the OH+(Ã 3ΠΩ)+H(n=1) channel, but is consistent with the vertical ionization energy to H2O+[(1b1)−2(4a1)1 2A1] that has been assumed to correlate with the above dissociation limit in the literature. The vibrational distribution of OH+(Ã 3ΠΩ) evaluated from the OH+(Ã 3ΠΩ,v′→X̃ 3Σ−,v″) band intensities is similar to the prior distribution in the rigid-rotor harmonic-oscillator approximation.