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Our interest in identifying magnetic nanoparticles for magnetic hyperthermia therapy has led to this study of the ac magnetic field induced heating of MnFe2O4 nanoparticles. Single-crystal MnFe2O4 nanoparticles with diameters ranging from 5.3 to 12.1 nm were synthesized by a thermal decomposition method. Oleic acid and oleylamine ligands used during synthesis were replaced by meso-2,3-dimercaptosuccinic acid, which allowed the particles to be dispersed in water. Magnetic field induction heating of aqueous dispersions of the particles showed that the value of specific absorption rate (SAR) increased with increasing particle size until it peaked for particles with an average diameter of 10.5 nm and then decreased for particles with an average diameter of 12.1 nm. Theoretical calculations of the effect of particle size on the heat generation were in agreement with this trend and showed that, with the largest particles, there was a decrease in the contribution of Brownian relaxation to the heating, leading to a decrease in the SAR for these particles.