Aging (or longevity) is one of the most important and potentially limiting factors in the use of nano-Fe0 to reduce groundwater contaminants. We investigated the aging of FeH2 (Toda RNIP-10DS) in water with a focus on changes in (i) the composition and structure of the particles (by XRD, TEM, XPS, and bulk Fe0 content) and (ii) the reactivity of the particles (by carbon tetrachloride reaction kinetics, electrochemical corrosion potentials, and H2 production rates). Our results show that FeH2 becomes more reactive between 0 and ∼2 days exposure to water and then gradually loses reactivity over the next few hundred days. These changes in reactivity correlate with evidence for rapid destruction of the original Fe(III) oxide film on FeH2 during immersion and the subsequent formation of a new passivating mixed-valence Fe(II)-Fe(III) oxide shell. The effect of aging on the rate of carbon tetrachloride reduction was best described by the corrosion potential of FeH2, whereas the yield of chloroform from this reaction correlated best with the rate of H2 production. The behavior of unaged nano-Fe0 in the laboratory may be similar to that in field-scale applications for source-zone treatment due to the short reaction times involved. Long-term aged FeH2 acquires properties that are relatively stable over weeks or even months.