The crystal structure, electronic properties, photocatalytic activity, and photocurrent response of a new antimony oxysulfide Sr2Sb2O2S3 and its oxyselenide analogue Sr2Sb2O2Se3 are presented. Both oxychalcogenides contain heteroleptic SbOQ(4) units with stereochemically active 5s(2) electron pairs. Our combined experimental and computational study highlights the structure-property relationships in this family of materials. By means of density functional theory calculations, we show very low effective masses for the electrons (m(e)* = 0.191(6) and 0.163(2) m(0)) and holes (m(h)* = 0.276(2) and 0.190(2) m(0)) for the oxysulfide and the oxyselenide, respectively, an indication of very high mobilities. Using DFT calculations, we attribute the low effective mass values (related to the curvature of the bands) to the nature and strength of the bonding between the lone pair electrons and the anions in the studied structure compared with other structure (Sr6Cd2Sb6S10O7). We analyze the states contributing to the lone pair stereoactivity and consequently to the observed photocurrent response and photocatalytic behavior under solar irradiation. This activity, the band gap values, and the band edge positions illustrate the potential of these antimony oxychalcogenides as promising candidates for water splitting using solar energy. Our study unlocks some key features in designing oxychalcogenides with low effective masses, which are advantageous for photocatalysis.