Using the Faraday impedance method, the diffusion coefficients of cobalt ions in a molten sodium borosilicate of equimolar composition with various CoO additives (0.3–3 wt %) in the temperature range 1073–1223 K were measured. It was shown that at AC frequencies in the range of 1–1000 Hz, the electrode process with the participation of cobalt ions proceeds in the diffusion mode. A monotonic decrease in the diffusion coefficients of cobalt ions with an increase in the concentration of cobalt oxide in the melt was revealed. This dependence is satisfactorily linearized in logarithmic coordinates. The dependences of ln D on 1/T are used to calculate the activation energies of cobalt ions diffusion. With an increase in the concentration of CoO, the activation energy of diffusion first increases (0.3–1.5 wt % CoO), and then decreases (2.5–3 wt %). Using the melt resistance measured at different temperatures, we estimated the activation energy of electrical conductivity. It is significantly less than the activation energy of diffusion and does not depend on the concentration of CoO in the melt. The obtained concentration and temperature dependences of the diffusion coefficients indicate the participation of cobalt ions in the polymerization and microstratification of the melt.