For a mixed oxide-ion and electron conducting oxide, with oxygen vacancies (V(O)) and electrons (e') or holes (h ) as charge carriers, a flux of (V(O)) (J(i)) can in principle be driven, not only directly by its own electrochemical potential gradient (inverted Delta eta(i)), but also indirectly by that of electrons (inverted Delta eta(e)), and vice versa for the flux of electrons (J(e)). It is common practice to assume that electrons and mobile ions migrate independently, despite the lack of experimental evidence in support of this. Here, all the Onsager coefficients, including the cross coefficients, have been measured for Ce(0.8)Pr(0.2)O(2-delta) within the a(O(2)) range 10(-21)-1 at 800 degrees C, using local ionic and electronic probes in a four-probe configuration. The cross coefficients of transport were found to be negligible in comparison to the direct coefficients in the a(O(2)) range 10(-21)-10(-4), but of the same order of magnitude as the direct coefficients for high a(O(2)) values (10(-2)-1). This is in contrast to the commonly used assumption that the two types of carriers migrate independently, i.e. that L(ie) = 0.