Thermoelectric power studies of polycrystalline materials of Sn/Nb substituted Mn-Zn ferrites have been carried out. The Seebeck coefficient, carrier concentration and drift mobility were evaluated. The Seebeck coefficient increases up to 420 K and 400 K for Nb and Sn substituted ferrites respectively. All the ferrites exhibited n-type conduction over the range of temperature studied. At high temperatures, a non-degenerate type of semiconductor behaviour has been observed. The conduction of these materials is explained by hopping of electrons from Fe2+ to Fe3+, in addition to the existence of narrow bands and localized levels. The variation of drift mobility is explained by wavy bands [1, 2].