Nanoparticles (1 nm-3 nm) of metallic nickel supported on alumina (4.3% Ni-17.9% Ni w/w) were prepared from a colloidal precursor inserted in an organic matrix. Their structural and stability properties have been studied by X-Ray Diffraction (XRD), Electron Paramagnetic Resonance (EPR) and Thermal Gravimetric Analysis (TGA). Benzene hydrogenation at atmospheric pressure in the temperature range of 75 degreesC-200 degreesC was used as a test reaction of their catalytic capability. The thermal stability of the particles depended on the nature of the reactive atmosphere. Thus, a growth in size (up to around 20 nm) is observed under H-2 flow at 350 degreesC or during benzene hydrogenation but not under air flow at 300 degreesC. The growth may be due to the coalescence of the metal particles during the heating and decomposition of the stabilizing organic matrix. Under oxidative atmosphere, stable nickel oxide particles, firmly attached to the support, are formed. The catalysts pre-treated under H-2/350 degreesC were active and stable in benzene hydrogenation. The observed activities depended on the reaction conditions and nickel composition.