A monoclinic distortion of tetragonal cell was found in the solid solutions with layered perovskite-like structure [Bi₅Nb_3–3xMn_3xO_15–δ (0.005 < х ≤ 0.06)]. In the ESR spectra absorption bands with g-factors ~4.3 and 2.1 assigned to Mn(II) and Mn(IV) atoms are recorded. The studies of magnetic susceptibility of the Bi₅Nb_3–3xMn_3xO_15–δ solid solutions revealed the formation of ferro- and antiferromagnetically bonded dimers of Mn(II), Mn(III), and Mn(IV) atoms. The exchange parameters in dimers and the distribution of paramagnetic manganese atoms in dependence to solid solution concentrations were calculated.