This research study was to elucidate the mechanisms of arsenic mobilization from mine tailings in the presence of natural organic matter (NOM). Humic acid (HA) was chosen as a model for NOM. The introduction of the HA at a low mass ratio (below 2mg added HA/g mine tailings) inhibited arsenic mobilization under acidic conditions. Arsenic mobilization increased with increasing mass ratio. When pH was above 7, the addition of HA enhanced arsenic mobilization significantly. A mobilization isotherm was developed to predict arsenic mobilization from the mine tailings in the presence of HA. It was indicated that HA sorption to the mine tailings was essential for arsenic mobilization. HA might enhance arsenic mobility through formation of aqueous complexes, competition for adsorption and electrostatic interactions. Capillary electrophoresis analyses indicated that arsenic redox reactions might not have a significant effect on arsenic mobilization in this study. The mobilization of co-existing metals could enhance arsenic mobilization by helping incorporating it into soluble complexes in the presence of HA.