There are presently many applications using nanofluids in thermal engineering. Some examples include the use of nanoparticles in conventional coolants to enhance heat transfer rate by increasing its thermal conductivity. Other applications include the sealing of bearing cases and sealing of rotary shafts. Even at low weight concentration, thermal conductivity increases significantly. In biotechnology, magnetic nanoparticles have been proposed for thermal treatment of tumor using nanoshells and alternating magnetic fields to generate heat in localized points. This paper evaluates the use of aqueous ferrofluid composed of MnxZni1-xFe2O 4 nanoparticles for cooling applications in the ambient temperature range. The use of ferromagnetic fluid for cooling applications represents an encouraging alternative to traditional methods; the fact that the fluid can be pumped with no moving mechanical parts, using the magnetocaloric effect, can be a great advantage for many applications where maintenance or power consumption are undesirable. A magnetic fluid suitable for this specific application has to have certain specific properties, like low Curie temperature, high magnetization, low viscosity and high specific heat. The selection of this ferrofluid is made based on its low Curie temperature (Tc), high saturation magnetization (Ms), low viscosity and high specific heat. The selection of a Mn-Zn ferrite-based aqueous ferrofluid was made based on its low Curie temperature compared with more commercially common magnetite-based ones. The synthesis of the ferrite nanoparticles was carried out by chemical precipitation and the process is described further on. Magnetic characterization of MnxZn1-xFe2O4 nanoparticles included the determination of Ms as a function of composition at 300K and the dependence of Ms with temperature for a specific 'x' value. Both types of measurements were carried out by using SQUID (Superconducting Quantum Interference Device) magnetometer. Copyright © 2005 by ASME.