Magnetization and AC susceptibility measurements have been performed on∼3 nm NiO nanoparticles in powder form. The results indicate that the structure ofthe particles can be considered as consisting of an antiferromagnetically orderedcore, with an uncompensated magnetic moment, and a magnetically disorderedsurface shell. The core magnetic moments block progressively with decreasingtemperature, according to the distribution of their anisotropy energy barriers,as shown by a broad maximum of the low field zero-field-cooled magnetization(MZFC) and in the in-phasecomponent χ’ of the ACsusceptibility, centred at ∼70 K. On the other hand, surface spins thermally fluctuate and freeze in a disorderedspin-glass-like state at much lower temperature, as shown by a peak inMZFC (at 17 K, forH = 50 Oe) and inχ′. The temperature ofthe high temperature χ′ peak changes with frequency according to the Arrhenius law; instead, for the lowtemperature maximum a power law dependence of the relaxation time was found,τ = τ0(Tg/(T(ν)−Tg))α, whereα = 8, like inspin glasses, τ0 = 10−12 s and Tg = 15.9 K. The low temperature surface spin freezing is accompanied by a strong enhancement ofmagnetic anisotropy, as shown by the rapid increase of coercivity and high fieldsusceptibility. Monte Carlo simulations for core/shell antiferromagnetic particles, with anantiferromagnetic core and a disordered shell, reproduce the qualitative behaviour of thetemperature dependence of the coercivity. Interparticle interactions lead to a shift to a hightemperature of the distribution of the core moment blocking temperature and to areduction of magnetization dynamics.