The limits of metal cation substitution and distribution in the sequence Bi2Fe4O9-Bi2Mn4O10 have been investigated by solid state synthesis, X-ray powder diffraction, and Mössbauer spectroscopy. Rietveld refinements conducted for the entire range along the join indicate the structures are orthorhombic with space group Pbam, with partial transition-metal site disorder confirmed and detailed by Mössbauer spectroscopy. Single-phase regions are found near each end-member and a two-phase region is observed at intermediate compositions, extending from about x=1 to 3, according to the general formula of the mixed crystals Bi2Fe4-xMnxO10-δ. An incorporation of Mn at octahedral sites replacing Fe is taken into account for the Bi2Fe4O9-related side of the system. Charge compensation is believed to be effected by addition of O, which gives rise to the formation of FeO5 pyramids. At the Bi2Mn4O10-related side of the system, substitution of pyramidal Mn3+ by Fe3+ is envisaged.