Iron nanoparticles supported on montmorillonite (MMT-Fe) were synthesized via the reduction by sodium borohydride of iron salts dissolved in a suspension of MMT. The MMT-Fe black powder collected after the evaporation of the solvent was analysed by Transmission Electron Microscopy, which revealed the formation of aggregates of metallic nanoparticles with an average size of 57 ± 17 nm dispersed on the surface of MMT. According to the X-ray diffraction, no iron ions are intercalated in the interlayer spacing of MMT, and no other crystalline species are formed. 57Fe Mössbauer spectroscopy evidences the formation of mainly zero valent iron in the form of iron boride. The O2 absorption kinetic of the synthesized powders was found to follow a second-order law. The study of the O2 absorption properties of as-synthesized, dried and stored (40 days) powders shows reaction constant (k), coefficient of proportionality (n) and O2 absorption capacities of the same order of magnitude. The O2 absorption capacity of the as-synthesized, dried and stored powders were found equal to 0.20 ± 0.01, 0.14 ± 0.03 and 0.09 ± 0.00 g O2 per g of iron, respectively. The initial absorption rate was found within the range [0.5–1.5]% O2 min−1 g−1.