Pure aluminum was implanted with 30 keV oxygen ions by-means of electron-cyclotron resonance plasma based ion implantation. Virgin and treated samples were investigated by Auger electron : microscopy and x-ray photoelectron spectroscopy to determine the depth distribution and chemical interactions of oxygen near the surface. The results are consistent with precipitation of oxygen and the formation of particles of Al2O3. Several samples were subsequently annealed at temperatures of up to 550 degrees C for 30 min. As-implanted and annealed specimens were characterized using an ultralow load nanoindentation test system. The results indicated an increase in aluminum hardness by a factor of 2 to 3; annealed samples were found to have lower hardness values than the as-implanted samples. A decrease of the experimental hardness with depth correlates with the concentration of implanted oxygen. This is consistent with the assumption that the improvement in strength is due to the formation of nanoprecipitates whose density increases with the oxygen implantation.