It is shown that the method of synthesis and source of bismuth greatly affect the physicochemical properties of modified MoVTeNbO catalysts. Adding an organic bismuth compound with a neutral pH at the stage of mixing the solutions of the initial components is accompanied by precipitation and the formation of inactive phases when the required M1 phase is missing. Regardless of the nature of the initial bismuth compound, introducing it into a dry MoVTeNb precursor leads to considerable destruction of it, resulting in a lower content of the M1 phase and the formation of binary V–Mo and Te–Мo oxides. It has been established that the ethylene yield attains a maximum (76.5 %) in the oxidative ethane dehydrogenation (OED)reaction, when the calcined MoVTeNb oxide catalyst is impregnated with a bismuth organic compound solution. The yield of acrylic acid is ≈48% over the same catalyst in the reaction of propane oxidation. Introducing bismuth nitrate into a solution of the initial components is nevertheless a more technologically advanced way of preparing bismuth-containing catalysts. The maximum ethylene yield is ≈76% in the OED reaction over the catalyst obtained using bismuth nitrate.