Magnetic susceptibility of the iron-containing solid solutions BiNb_1−xFe_xO_4-δ in orthorhombic and triclinic modifications has been studied. The reversibility of the phase transition from the high-temperature triclinic modification to the orthorhombic one has been revealed by means of magnetic dilution and X-ray phase analysis by the example of polycrystalline samples of the solid solutions BiNb_1−xFe_xO_4-δ.