Magnetic resonance has been used to obtain the surface anisotropy in manganese–ferrite nanoparticles from 100 to 250 K. The surface anisotropy data has been successfully explained using a model that includes the single-ion (magnetocrystalline) term plus the magnetoelastic term The single-ion surface-anisotropy contribution has been calculated using the mean-field approximation proposed by Millev and Fänhle [Phys. Rev. B 51 (1995) 2937]. The temperature dependence of the magnetostriction has been included in the description of the magnetoelastic term. We found that the surface-anisotropy changes from negative to positive at a characteristic temperature which is explained by the model proposed in this study.