Magnetic susceptibility and ESR properties of manganese-containing solid solutions of bismuth orthoniobate BiNb_1–xMn_xO_4–δ have been studied. Mn(II), Mn(III), and Mn(IV) atoms as well as their dimers exhibiting antiferro- and ferromagnetic types of exchange have been found in the solid solutions. Parameters of the exchange interactions in the clusters and the distribution of monomers and dimers of manganese atoms in the solid solutions have been calculated as functions of the paramagnetic atom fraction.