Single-crystal sillenite-type Bi-24(M2-xMnx4+)O-40 (M = Al3+, Fe3+, Ge4+, Ti4+, Cr4+, V5+) solid solutions have been studied by a complex of methods: single crystal and powder X-ray diffraction; EXAFS/XANES, IR and Raman spectroscopies; scanning and transmission electron microscopies; electron probe X-ray microanalysis. Structures of Bi-24(M2-xMnx)O-40 (M = Al, Fe, V, Ge) solid solutions contain Mn ions with two formal charges, Mn4+ and Mn5+, as well as vacancies in the Bi (M = V) and O (M = Cr, Fe, Al, V) sites. For the Bi-24(M2-xMnx)O-40 (M = Fe, Cr, V), a partial occurrence of Bi atoms in the tetrahedral sites has been detected by X-ray diffraction, which was confirmed by IR and Raman spectroscopies. Two isostructural sillenite-type solid solutions based on the Bi24Mn2O40 and gamma-Bi2O3 with the impurity phases Mn2O3 and Bi2Mn4O10 were found in the system with M = Cr. Limits for isomorphic substitutions in the Bi-24(M2-xMnx)O-40 (M = Al, Fe, Ge, Ti, Cr, V) systems were theoretically estimated and compared with the experimental results obtained. A potential of elaborated combination of methods aimed at determining the precise real composition of solid solutions not identical to that of the initial charge was demonstrated. Homogeneity regions for sillenite family phases were predicted.