Abstract Surface area-normalized rate constants ( k SA) of reaction between metallic nanoparticles and reducible contaminants, such as chlorinated hydrocarbons, heavy metals, and nitrate, have been reported to be dramatically increased as compared to that of commercial metallic powder. However, k SA for individual pollutants in previously published data vary by as much as 1–2 orders of magnitude and much of this variability is due to the effect of various sizes. The size dependence of the reactivity of nanoparticles is not yet fully understood; however, yielding nanoparticles with uniform size and without agglomeration during the period of reaction would demonstrate the effect of varying particle size. In this study, resin-supported zerovalent copper with average particle size of 7, 10, 18, 26, and 29, respectively, were successfully synthesized and evidenced no agglomeration during the reaction period of 10 h. The k SA of copper nanoparticles ( k n,SA) was 110–120 times higher than that of powdered copper particles ( k p,SA) when the copper particle size was about 10 nm. However, for diameters of 18–29 nm, the ratio of k n,SA/ k p,SA was around 10–20, indicating that the reactivity of small copper nanoparticles (∼10 nm) varies discontinuously. Thus, most variability in previous k SA is attributed to the presence of small nanoparticles.