Electrical conductivity measurements at different temperatures (298-598 K) and frequency of 500 kHz were made on the systems Ni1-xZnxFe2O4 (x = 0.1, 0.3, 0.5, 0.7 and 0.9) and Ni0.6Zn0.4CryFe2-yO4 (y = 0.00, 0.05, 0.10, 0.15, 0.20 and 0.25). The investigated samples were prepared by the standard ceramic method and sintered at 1250degreesC. The ferrite samples were crystallized in the spinel phase. The electrical conduction in these ferrites is explained on the basis of the hopping mechanism. The composition dependence of the conductivity is explained in terms of the cation valences and distribution. The activation energy of each sample is calculated from the plots of log sigma vs. 10(3)/T at high temperature region. (C) 2003 Elsevier B.V. All rights reserved.