Ferromagnetic resonance (FMR) experiments were performed as a function of temperature (10–300K) on γ-Fe 2 O 3 nanoparticles prepared by a sol–gel method. By measuring at several temperatures the relative intensity of the spectrum due to superparamagnetic particles and the anisotropy field of the spectrum due to ferrimagnetic particles, we determined the size distribution of γ-Fe 2 O 3 nanoparticles. It was found to be a log-normal distribution with a most probable diameter D m =8.1nm and a standard deviation σ=0.25. Transmission electron microscopy measurements performed on the same samples yielded a log–normal distribution with D m =11.2nm and σ=0.23. The difference is attributed to the existence of a disordered surface layer in the particles.