High-surface area activated carbons (HAC) were prepared from analogous sulfur-containing precursors by using KOH chemical activation. The relationship between pyrite (FeS2) in the precursor and the pore structure of the as-prepared HAC was investigated comprehensively. The results suggest that FeS2 in the precursor reacts with KOH, producing K2SO4, K2S2O3, K2S, thioether (R-S-R'), and Fe3O4, which reduce the actual amount of KOH used to develop the porosity. As a result, the existence of the by-product Fe3O4 further clarifies that FeS2 in the precursor can consume some amount of KOH, which leads to the decrease in the specific surface area and pore volume of HAC. Interestingly, part of the inorganic sulfur from FeS2 in the precursor can be transformed into organic sulfide R-S-R'.