Arsenic is one of the heavy metals and nearly all its compounds, especially organic compounds, are toxic. The wide spectrum of diseases caused by this element has led to evaluation of the toxicity of different arsenic species and identification of the major natural and anthropogenic pollution sources of it in the nature. Mining activities are among the main sources of anthropogenic pollution of soil and water by arsenic. The purpose of this study was geochemical modeling of different arsenic species in the wastewater of the tailings dam of Mouteh Gold processing plant in Iran to evaluate the effect of pH and temperature on the stability of these components. Modeling was done using MINTEQ software. The results showed that arsenic species at different pH values under study were H3AsO3, H2AsO3- and HAsO32-, and their actual concentration in the plant wastewater were negligible. MINTEQ software introduced H3AsO4, H2AsO4-, HAsO42- and AsO43- as arsenic V species at different pH values, of which HAsO42- and AsO43- were the main components of arsenic in plant wastewater. Given the low toxicity of arsenic V species and their easier elimination relative to arsenic III species, in the current conditions, the plant wastewater is in a good status in terms of arsenic pollution. Also temperature changes have little effect on the concentration of various arsenic species in the wastewater.