We report the crystal structure and electron density of samarium titanium oxysulfide, Sm(2)Ti(2)S(2)O(4.9), photocatalyst obtained through the Rietveld analysis, maximum-entropy method (MEM) and MEM-based pattern fitting of the high-resolution synchrotron powder diffraction data taken at 298.7 K. The Sm(2)Ti(2)S(2)O(4.9) has a tetragonal structure with the space group I4/mmm. Refined occupancy factors at the ;equatorial' O1 and ;apical' O2 sites were 0.994 (3) and 0.944 (12), respectively, which strongly suggest oxygen deficiency at the O2 site. Electron-density analyses based on the synchrotron diffraction data of Sm(2)Ti(2)S(2)O(4.9) in combination with density-functional theory (DFT) calculations of stoichiometric Sm(2)Ti(2)S(2)O(5) reveal covalent bonds between Ti and O atoms, while the Sm and S atoms are more ionic. The presence of S 3p and O 2p orbitals results in increased dispersion of the valence band, raising the top of the valence band and making the material active at visible wavelengths. The present DFT calculations of stoichiometric Sm(2)Ti(2)S(2)O(5) indicate the possibility of overall splitting of water, although Sm(2)Ti(2)S(2)O(4.9) works as a visible-light-responsive photocatalyst in aqueous solutions only in the presence of sacrificial electron donors or acceptors. The oxygen deficiency and cocatalyst seem to be factors affecting the catalytic activity.