Superparamagnetic nanoparticles of magnetite (Fe3O4) 2 ran in size were produced by a co-precipitation method. Superparamagnetic resonance (SPR) spectra at room temperature show a broad line with a Lande g-factor, g(eff) approximate to 2. It was observed that, as the temperature decreased to 24 K, the apparent resonance field decreases while the line width considerably increases. We used a theoretical formalism based on a distribution of diameters or volumes of the nanoparticles. The nanoparticles behave as single magnetic domains with random orientations of magnetic moments which are subject to thermal fluctuations. A Landau-Lifshitz line shape function presents adequate results which are in good agreement with the experimental ones. A single set of parameters provides good fits to the spectra recorded at different temperatures. At high T the SPR line shape is governed by the core anisotropy and the thermal fluctuations. By decreasing the temperature, the magnetic susceptibility of shell spins increases. As a result of this, the surface spins produce an effective field on the core leading to a decrease of resonance field, B,. Also, the effective anisotropy increases as the shell spins begin to order. So, the results are interpreted by a simple model, in which each single-domain nanoparticle is considered as a core-shell system, with magnetocrystalline anisotropy on the core and surface anisotropy on the shell. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.