Magnetic susceptibility and ESR properties of iron-containing Bi₂BaNb_2–2xFe_2xO_9–δ solid solutions with layered perovskite-like structure have been studied. In the diluted solid solutions, iron atoms are combined into Fe(III) dimers and tetramers with antiferro- and ferromagnetic exchange types. The exchange parameters and the distribution of aggregates in the solid solutions have been calculated as functions of the paramagnetic atoms fractions.