X-ray diffraction, Infra-red spectroscopy and scanning electron microscopy have been used to investigate the Mn0.5Ni0.1Zn0.4AlxFe2 -xO4 system. The analysis 0.15 in steps of 0.025). The analysis of IR spectra indicates the distribution of Al-ions between both A and B-sites. The electrical resistivity and thermoelectric power as a function of temperature and Al-content have been investigated. It is observed that the resistivity increases with increasing the Al-content. It is also found that the temperature variation of resistivity exhibits two breaks, each break is associated with a change in the activation energy. Measurement of the thermoelectric power reveals n-type conduction for all samples. The activation energies of all samples in the paramagnetic region are found to be greater than those in the ferrimagnetic region. The results are explained on the basis of the formation of spin polarons in the paramagnetic region. (c) 2005 Springer Science + Business Media, Inc.