X-ray diffraction studies employing the Rietveld analysis is reported on the influence of increasing Co substitution on the structural and microstructural evolution in AB(2)O(4) type spinel ferrites: Ni1-xCoxFe2O4 (x = 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0). The specimens were synthesized by the organic precursor method. Structure refinement reveals that the pure NiFe2O4 is not an exact inverse spine] and about 11% of Ni2+ ions occupy the tetrahedral (A) sites. Increasing Co concentrations had the effect of increasing the ratio of [Fe3+](B)/[Fe3+](A) and gradual expansion of the ferrite unit cell. The microstructure refinement estimates that the particle size values are in the order of similar to nm, ranging from 31 to 61 nm, which gradually increase with increasing Co doping accompanied by almost negligible lattice micro strains (similar to 10(-4)). The corresponding particle size distribution for each specimen was obtained from the X-ray diffraction data from the basic assumption that the spherical nanoparticles follow the log-normal distribution. The size distribution for the pure NiFe2O4 was also estimated from transmission electron microscope and agreed well with that obtained by the diffraction data analysis. (C) 2011 Elsevier B.V. All rights reserved.