Rutherford backscattering spectrometry, thermogravimetric analysis, and high-resolution transmission electron microscopy were used to investigate the oxidation behavior of ultrafine grain aluminum powder. Fractional change in mass of Al powder samples were obtained as a function of temperature and exposure time for samples with different particle size distributions. As expected from surface energy considerations, the activation energy for oxidation of ultrafine grain particles is less than that for nominally flat surfaces. Activation energy for oxidation of powder samples with average particle diameters from 240 to 650 Å was determined to be 0.5 eV, which is smaller than the value of 1.7 eV known for flat Al samples.