The electrotransport behaviour of matrix and solute cations in a doped transition metal oxide (A(1-u)B(alpha))(1-delta)O is analyzed. A microscopic model is presented to describe the transport coefficients of the charge carriers (cations and electron holes) on the basis of short range and long range interactions between the mobile defects of the system. The model calculations show that the apparent effective charge of the solute ions may become negative if strong interactions between vacancies and solute ions arise.