We analyze the demixing behavior of heterovalent doped transition metal oxides, (A, B)O, in an applied oxygen potential gradient, using a defect model for the doped oxide and the five-frequency model for impurity diffusion. Experimental results for the demixing of Ga-doped cobalt oxide, (Co1-xGax)O, where the dopant becomes enriched on the low-oxygen-potential side, yield in combination with cation tracer diffusion coefficients of Co and Ga an impurity-vacancy binding energy of about 0.4 eV and a ratio of the non-diagonal and the diagonal coefficients in the Onsager transport matrix L(CoGa)/L(GaGa) = -1.6