Zerovalent iron nanoparticles (Fe(0) NPs or nZVI) synthesized by reductive precipitation in aqueous solution (Fe/FeO) differ in composition and reactivity from the NPs obtained by reductive precipitation in the presence of a S-source such as dithionite (Fe/FeS). To compare the redox properties of these types of NPs under a range of environmentally relevant solution conditions, stationary powder disk electrodes (PDEs) made from Fe/FeO and Fe/FeS were characterized using a series of complementary electrochemical techniques: open-circuit chronopotentiometry (CP), linear polarization resistance (LPR), electrochemical impedance spectroscopy (EIS), and linear sweep voltammetry (LSV). The passive films on these materials equilibrate within minutes of first immersion and do not show further breakdown until >1 day of exposure. During this period, the potentials and currents measured by LPR and LSV suggest that Fe/FeS undergoes more rapid corrosion and is more strongly influence by solution chemical conditions than Fe/FeO. Chloride containing media were strongly activating and natural organic matter (NOM) was mildly passivating for both materials. These effects were also seen in the impedance data obtained by EIS, and equivalent circuit modeling of the electrodes composed of these powders suggested that the higher reactivity of Fe/FeS is due to greater abundance of defects in its passive film.