Here we show that the Onsager cross terms for ion–electron interactions are not an artifact, but the necessity to phenomenologically and completely describe the mass/charge transport of a mixed ionic–electronic conductor in terms of mobile charged components which are the only experimentally operable species. The use of an appropriate comprehensive defect model may help to reduce the cross terms (which depend on the choice of formal charge of the mobile defects), but it cannot obviate them if long-range Coulombic interactions are in action among the defects.