Exposing nanoscale zerovalent iron (NZVI) to dissolved sulfide species improves its performance as a remediation agent. However, the impacts of sulfur dose and sulfidation time on morphology, sulfur content, reactivity, and selectivity of the resulting sulfidized NZVI (SNZVI) have not been systematically evaluated. We synthesized SNZVI using different sulfur doses and sulfidation times and measured their properties. The measured S/Fe molar ratio in the particles ([S/Fe]particle) was 10-500 times lower than [S/Fe]dosed but was predictable based on [S/Fe]dosed × tsulfidation. The low sulfur content (0.02-0.65 mol % S/Fe) inhibited the reaction of SNZVI with water (up to 13-fold) and increased its reactivity with trichloroethene (TCE) (up to 14-fold) and its electron efficiency (up to 20-fold). A higher [S/Fe]particle (0.86-1.13 mol % S/Fe) led to complex particle structures and lowered the resistance to electron transfer but did not improve the benefits realized at the lower S/Fe ratios. Adding small amounts of sulfur into NZVI led to more accumulation of acetylene, especially for low Fe/TCE conditions, suggesting that sulfur lowers the rate of hydrogenation of acetylene to ethene. These results show that [S/Fe]dosed × tsulfidation can be used to predict the measured S content in the particles and that affects reactivity, longevity, and electron selectivity, for post-SNZVI.