Phase equilibria in the NiO-Nb2O5 and SrCO3-NiO-Nb2O5 systems are studied by physicochemical methods. The formation of the Ni4Nb2O9 phase is confirmed. The extents of the (Sr1-xNix)4Nb2O9 and (Sr1-xNix)6Nb2O11 solid solutions are determined. The extents of phase fields for solid solutions with a perovskite-related structure are established using the criterion of tolerance and the ratio of electronegativities. The major parameters of the synthesis of the above solid solutions are found by isothermal thermogravimetry invoking formal kinetic analysis.