Zn ferrite nanoparticles ranging from 3 to 19 nm have been obtained under solvothermal conditions. In addition, two samples of ZnFe2O4/SiO2 nanocomposite of 11 and 20 nm ferrite particle size have been also synthesized by means of the sol gel method. To evaluate the inversion degree of these ferrite samples with mixed spinel structure ((Zn1-xFex)[ZnxFe2-x]-O-4), X-ray absorption spectroscopy measurements at the Zn and Fe K-edge have been performed, and the obtained spectra along with the Fourier transform of the EXAFS signal have been compared to those corresponding to a ceramic sample with a lower inversion degree. All the samples composed by nanosized particles illustrate superparamagnetic behavior above the blocking temperature as it can be inferred from the high values of ZFC and FC magnetic susceptibility that they present. Mossbauer measurements done to the different nanosized particles obtained by the solvothermal method were useful to study the "frozen" to "superparamagnetic" crossover with the thermal variation and estimate the anisotropy constant that has been found to increase as the particle size decreases.