The properties of the La0.45Ba0.55CoO3–x cobaltites have been investigated as a function of the oxygen content. It has been shown that a decrease in the oxygen content leads to a decrease in the spontaneous magnetization, as well as to a metamagnetic behavior and a sharp increase in the magnetoresistance effect. The metamagnetism and magnetoresistance effect are caused by the phase transition from the antiferromagnetic state to the ferromagnetic state in an external magnetic field. It has been assumed that the insulating antiferromagnetic phase contains cobalt ions in a mixed high-spin/low-spin state, whereas the conducting ferromagnetic phase includes these ions in an intermediate spin state.