Magnetic nanoparticles with different compositions (CoxFe3<xO4, 0<x<0.1) were synthesized from metal salts using a coprecipitation technique to produce magnetic fluids following a peptization technique. The liquid carrier was the hydrocarbon Isopar M and the surfactant was oleic acid. The colloidal-sized ferrimagnetic nanoparticles produced were found to be superparamagnetic. Measurements of the complex magnetic susceptibility were carried out to evaluate the resonant frequency fres, the anisotropy constant K, and anisotropy field HA. fres was found to be a linear function of the cobalt content of the magnetic nanoparticles over the range of cobalt content studied.