Magnetic measurements on small γ-Fe2O3 particles (diameter D∼3nm) have been carried out. Features in the zero-field-cooled and field-cooled magnetization data and the high-field irreversibility in the low-temperature hysteresis curve are found to be consistent with a collective thermal blocking process. The dynamical properties, studied by ac magnetic susceptibility measurements, suggest the occurrence of a spin-glass like transition with a critical exponent zv 8.00.8. This finding is also supported by the H2/3-dependence of the temperature transition (Tg) which is consistent with the Almeida-Thouless critical line (δTg ∼H2/3) of canonical spin glasses. © 2010 IOP Publishing Ltd.