Electrodeposited nickel and nickel-yttrium oxide composite samples were studied in situ using scanning electrochemical microscopy (SECM). The monitored probe currents in phosphate-citrate buffer (pH 4.2) in the presence or absence of Ru(NH3)(6)(3+) as an oxidizing mediator near the Ni surface show that the SECM is a useful tool for studying the electrochemical activity of heterogeneous metal surfaces at micrometer scales. The SECM ultramicroelectrode probe tip provides information about the shape, activity, and location of particles, such as Y2O3 introduced (codeposited) in the Ni matrix of the composite. Experiments show that the Ni matrix in the composite coating is more active than the pure Ni coating. This fact is expected, because of texture changes in the Ni structure upon introduction (by codeposition) of Y2O3 particles. In the absence of a mediator in the solution, the electrochemical activity of heterogeneous metal surface at a microlevel is investigated by using O-2 concentration changes. The rate of reaction for O-2 reduction was found to vary locally at electrodes floating at the open-circuit potential (OCP) when compared to an electrode potentiostatically polarized at a more positive potential than the OCP. This behavior suggests that local anode and cathode regions are being observed at the OCP sample.