We report the synthesis and the crystal and electronic structure as well as the optical and photocatalytic properties of novel photoactive materials of the general formula Bi26-xMgxO40. Two compounds with compositions of Bi24.28(3)Mg1.72(3)O40 and Bi24.05(3)Mg1.95(3)O40 are synthesized using the pyrolytic method. According to X-ray diffraction analysis, the materials are monocrystalline species. Their electronic bandgaps determined from Tauc plots are 2.41 eV [Bi24.28(3)Mg1.72(3)O40] and 2.69 eV [Bi24.05(3)Mg1.95(3)O40]. Keeping in mind that optical bandgaps are typically larger than their electronic counterparts, we find that the bismuthate bandgaps match well that of Bi24Mg2O40 (2.26 eV) predicted by density functional theory. Apparently, the synthesized bismuthates are indirect bandgap semiconductors just like Bi24Mg2O40. Both materials demonstrate nearly identical luminescence spectra. Their luminescence emission at 620 nm is most efficiently excited by 365 nm light. The materials' photocatalytic properties are evaluated in a visible light-induced photocatalytic phenol degradation reaction. Rather low activity of both compounds is detected. However, Bi24.05(3)Mg1.95(3)O40 is ∼2 times more photocatalytically active than Bi24.28(3)Mg1.72(3)O40, which is associated with a higher Bi5+ content in the former.