Nanosize ZnxNi1-xFe2O4 spinel composites with x = 0, 0.2, 0.4, 0.6, 0.8 and I were synthesized by using surfactant (polyethylene glycol (PEG)) assisted hydrothermal route and characterized by TEM, XRD and VSM techniques. The crystallite size was calculated from different characterization methods, and magnetic core size was found to be in the range of 9-20 nm from VSM. All particles showed superparamagnetic character at room temperature and M, decreased with increasing concentration of Zn2+. Due to the bigger ionic radius of Zn2+ with respect to Ni2+, the unit cell parameter 'a' increased linearly with increasing x, likewise, the oxygen positional parameter 'u' increased theoretically and experimentally as observed in the literature. Particle size was observed to decrease by substitution of Zn. The cation distribution has been calculated analytically by using X-ray diffraction data and Fe3+ cations were found to occupy mostly tetrahedral sites revealing almost an inverse-spinel structure. These results are proved to be consistent with the results of magnetic measurements. The site preference of Fe3+ cations on tetra sublattice is attributed to the synthesis conditions utilizing surfactant and low temperature. (c) 2009 Elsevier B.V. All rights reserved.