The reactivity of particulate zero-valent metals in solution is affected by the metal type (e.g., Fe vs. Zn), particle size (nano vs. micro), surface conditions (passivation by coatings of oxides), and solution conditions (including the type and concentration of oxidants). Comparing the reactivity of various types of Fe0 and Zn0 with carbon tetrachloride (CCl4) shows that the intended effect of properties engineered to give enhanced reactivity can be obscured by effects of environmental factors. In this case, rates of CCl4 reduction by Zn0 are more strongly affected by solution chemistry than particle size or surface morphology. Under favorable conditions, however, Zn0 reduces CCl 4 more rapidly-and more completely-than Fe0, regardless of particle size. The suitability of nano-sized Zn0 for environmental remediation applications is uncertain.