The study investigates the reactivity of two types of nano-sized zero-valent iron (Fe0) particles, synthesized by different methods, for groundwater remediation. The first type, FeBt, is a two-phase material consisting of α-Fe0 and Fe3O4, while the second type, FeBt, is primarily composed of 20–80 nm metallic Fe particles with an oxide shell. Both materials exhibit more negative corrosion potentials than nano-sized Fe3O4, Fe3O4, micro-sized Fe0, or a solid Fe0 disk. They react more rapidly with benzoquinone and carbon tetrachloride compared to micro-sized Fe0, but surface area-normalized rate constants do not show a significant nano-size effect. The distribution of products from carbon tetrachloride reduction is more favorable with FeBt, producing less chloroform. The study also addresses methodological issues, such as the determination of relevant surface area for highly reactive nano-sized materials and the reproducibility of electrochemical experiments. The results suggest that the high reactivity of nano-Fe0 is primarily due to its large surface area, and that nano-Fe0 can significantly influence the distribution of products formed from contaminant degradation.The study investigates the reactivity of two types of nano-sized zero-valent iron (Fe0) particles, synthesized by different methods, for groundwater remediation. The first type, FeBt, is a two-phase material consisting of α-Fe0 and Fe3O4, while the second type, FeBt, is primarily composed of 20–80 nm metallic Fe particles with an oxide shell. Both materials exhibit more negative corrosion potentials than nano-sized Fe3O4, Fe3O4, micro-sized Fe0, or a solid Fe0 disk. They react more rapidly with benzoquinone and carbon tetrachloride compared to micro-sized Fe0, but surface area-normalized rate constants do not show a significant nano-size effect. The distribution of products from carbon tetrachloride reduction is more favorable with FeBt, producing less chloroform. The study also addresses methodological issues, such as the determination of relevant surface area for highly reactive nano-sized materials and the reproducibility of electrochemical experiments. The results suggest that the high reactivity of nano-Fe0 is primarily due to its large surface area, and that nano-Fe0 can significantly influence the distribution of products formed from contaminant degradation.