Abstract: The solvent extraction of thioarsenite by tri-n-octylmethyl-ammoniumchloride (TOMAC) of carbonate type is proposed for the separation of arsenic in alkali-leaching solutions of As-containing smelting dusts. The simulative leaching solutions are pretreated by sulfidation prior to solvent extraction. The maximum extraction capacity is determined and the extraction mechanism is also discussed. The influences of organic phase composition, organic-aqueous phase ratio, extraction time and temperature on As^Ⅲ extraction are further studied. The experimental results show that 85.3% of As^Ⅲ can be extracted with single stage extraction under the following conditions: the organic phase, 30% CO₃²-type TOMAC+15% Octanol+ 55% sulfonated kerosene; the water phase, 0.5 mol/L NaOH+9.69×10^-2mol/L As^Ⅲ; the room temperature; the phase ratio of V_O/V_A=1:1. After four-stage counter current extraction, the As^Ⅲ concentration of the rafinate is less than 1.34×10^-3mol/L while the extraction is more than 98%. The separation of arsenic in alkaline solutions can be effectively achieved.