The reactivity of FeS-coated iron nanoparticles (nFe/FeS) toward trichloroethylene (TCE) reduction was examined in both synthetic and real groundwater matrices to evaluate the potential performance of nFe/FeS in field treatment. The rate of TCE reduction increased with increasing pH, which is consistent with the pH effect reported previously for iron sulfide systems, but opposite that has been observed for (nonsulfidic) Fe-0 systems. The rates of TCE reduction were unaffected by ionic strength over the range of 0.1-10 mM NaCl, increased with Ca2+ or Mg2+ concentrations, and inhibited by the presence of humic acid. The inhibitory effect of humic acid on the reactivity of nFe/FeS was largely alleviated when humic acid was combined with Ca2+/Mg2+, Presumably due to decreased adsorption of humic acid onto nFe/FeS surface by the formation of humic acid-Ca2+/Mg2+ complexes.