This communication challenges the concept that nanoscale metallic iron (nano-Fe-0) is a strong reducing agents for contaminant reductive transformation. It is shown that the inherent relationship between contaminant removal and Fe-0 oxidative dissolution which is conventionally attributed to contaminant reduction by nano-Fe-0 (direct reduction) could equally be attributed to contaminant removal by adsorption and co-precipitation. For reducible contaminants, indirect reduction by adsorbed Fe-II or adsorbed H produced by corroding iron (indirect reduction) is even a more probable reaction path. As a result, the contaminant removal efficiency is strongly dependent on the extent of iron corrosion which is larger for nano-Fe-0 than for micro-Fe-0 in the short term. However, because of the increased reactivity, nano-Fe-0 will deplete in the short term. No more source of reducing agents (Fe-II, H and H-2) will be available in the system. Therefore, the efficiency of nano-Fe-0 as a reducing agent for environmental remediation is yet to be demonstrated. (c) 2010 Elsevier By. All rights reserved.