Temperature-sensitive Fe, Mn, Zn, Ni, Co. Cu and Ca ultrafine polyferrite magnetic fluids (MF) are produced for heating and cooling of MF seals and energy conversion systems. The thermomagnetic properties of the MF samples are investigated under the fields up to 2 T at temperatures ranging from 280 to 433 K using a vibration sample magnetometer. A long-term thermal treatment in the inert atmosphere at a relatively low temperature (∼500 K) improves the thermomagnetic properties of MF. With the increasing of the thermal treating time, the magnetization dependence on the temperature becomes more evident. After thermal treatment the magnetization and the pyromagnetic coefficient of MF increase.