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In this study, dynamic susceptibility was used to investigate magnetite nanoparticles surface-coated with dextran and dimercaptosuccinic acid and dispersed as biocompatible magnetic fluids. Transmission electron microscopy data have been used to support the analysis carried out with the susceptibility data. The field dependence of the peak position of the imaginary component of the dynamical susceptibility was analyzed within the picture of an asymmetric double well potential for the relaxation of the magnetic moment associated to the magnetite nanoparticle. The size-dependence of the magnetic susceptibility was taken into account in the data analysis. Nanoparticle-size parameters obtained from the analysis of the susceptibility data of the two samples (3.7 and 6.1 nm) are in very good agreement with the values obtained from the fitting of the electron microscopy data (3.1 and 5.6 nm).