The optimization of nanoscale zero-valent iron (nZVI) for groundwater remediation applications requires consideration of properties that influence its longevity and transport in porous media and reactivity with contaminants. Here, we report on the stabilization of nZVI by controlled growth of oxide shells of varying thickness and characterization of the resulting materials' structure and reactivity. Using a thermal oxidation method, nZVI was prepared with shell thickness varying between 4 and 10 nm. These nZVI materials, together with pyrophoric nZVI (without a passivating oxide coating) and two commercial nZVI materials (NANOFER STAR and NANOFER 25), were characterized in detail with respect to morphology, shell thickness, structure, magnetism, stability, and reactivity. The results show that increasing oxidation temperature results in thicker oxide coatings on the particles, but these coatings also have more fractures and other defects. The reactivity of these particles, demonstrated on Cr(VI) and Cu(II) removal, increases with increasing shell thickness, probably as a result of higher extent of defects in thicker shell. Therefore the ability to control thickness and character of the shell leads to possibility to controlling reactivity while keeping comparable content of Fe(0) in the material. These nZVI materials with 7 and 10 nm oxide shell prepared via simple solid-gas synthesis can be used as a suitable alternative to common air-stable nZVI without additional activation steps.