The conduction mechanism in Mg2+ and Al3+ substituted Li0.5Fe2.5O4 with general formula MgxAl2x Li0.5(1- x)Fe2.5(1- x) O-4 (x = 0.0, 0.2, 0.5, 0.6 and 0.7) has been studied by means of compositional and temperature dependent d.c. resistivity, thermoelectric power and I - V characteristics measurements. It is found that ferrites are electronic conductors. For x = 0.0 and 0.2 conduction is due to holes, while for x = 0.5, 0.6 and 0.7 it is due to electrons. Thermal variation of mobilities and activation energies determined through d.c. resistivity measurements confirm the formation of small polarons. The sample with x = 0.0 exhibits switching phenomena. C (c) 2005 Springer Science + Business Media, Inc.