Co-Ti ferrite (Co1+xTixFe2-2xO4, 0.2 <= x <= 0.5) nanoparticles with average diameters ranging from 6 to 12 nm were produced by a wet chemical method. The imaginary parts of the AC magnetic susceptibility chi", which is the most dominant value dependent on the thermal energy generated in the magnetic nanoparticles, were measured for samples placed in a 1-Oe, 100-Hz AC magnetic field at room temperature. The temperature corresponding to the peak of chi" decreased as the value of x increased. The heating mechanisms of the samples were studied on the basis of the dependence of the temperature increase on the magnetic field strength and frequency in the case of the samples with diameters of 12 nm. A temperature increase of approximately 10K was observed in a 300-Oe, 10-kHz field. The rate of increase in temperature Delta T/dt was determined to range from 0.001 to 0.008 K/s depending on the frequency and composition. AC magnetic susceptibility measurements revealed that the optimum concentration of Ti ions was found to be that corresponding to x = 0.3. These Co1+xTixFe2-2xO4(0.2 <= x <= 0.5) samples are expected to be used as thermal agents in magnetic hyperthermia treatment. (C) 2011 Elsevier B.V. All rights reserved.