Arsenic-calcium residue (ACR) is a hazardous solid waste generated by the metallurgical industry, posing a significant environmental risk. However, the stability and transformation behavior of ACR in sulfidic conditions remains unclear. Herein, we have investigated the stability and speciation evolution of arsenic (As), sulfur (S), and trace metals during the exposure of ACR to diverse S(-II) concentrations under anoxic conditions at pH of 6 and 11. Our results indicate that environmentally relevant levels of S(-II) (i.e., 1, 10, and 50 mM) significantly enhance the mobilization of As(III) and Cd