Adsorption of cetylpyridinium chloride (CPC), a cationic surfactant, on pyrite surface was investigated in its suspension. Maximum adsorption capacity of pyrite for CPC was 357 mmol kg^(-1) in pyrite suspension (4 g L^(-1)) at pH 7 equilibrated with CPC (0.1-1.8 mmol L^(-1)). As CPC adsorption proceeded in the suspension at pH < 7, zeta potential (z) of pyrite surface showed increasing positive values (i.e. 5-18.5 mV at pH 5 and 8.12-26 mV at pH 6). However, at pH ≥ 7, it changed from negative to positive value (i.e. -5 to 36 mV at pH 7, -10 to 30 mV at pH 8, and 15 to 20 mV at pH 9), indicating three different isoelectric points at each pH. Adsorption isotherms at suspension pH 5 and 7 with NaCl (0.01 and 0.1 M) showed a decreasing pattern in CPC adsorption capacity. Zeta potential of pyrite surface changed from positive to negative value by the addition of NaCl (21 to -16 mV at pH 5 and 5 to -22 mV at pH 7). This study provides basic understanding of the adsorption mechanism of cationic surfactant on pyrite surface at different pH conditions when surfactant wastewater and contaminated groundwater need to be treated by the reactive iron sulfide.