This study was conducted to investigate arsenic (As) readsorption phenomenon in acid-treated soil using phosphate as a competing ion. Three field-aged soils (i.e., 51, paddy soil; S2, field soil; 53, forest soil) originally contaminated with As ranging from 30 to 59 mg/kg-soil were collected from a former smelter site. When 0.2 M hydrochloric acid (HCl) alone was used as an extraction solution, As bound to iron (Fe) oxides was removed but significant amount of the released As was readsorbed onto residual Fe-oxides, yielding low As extraction efficiency of 11-27%. Readsorption of the released As seemed to occur preferentially on amorphous Fe-oxides. In contrast, As extraction efficiency was greatly increased by 0.2 M HCl solution supplemented with monopotassium phosphate (KH2PO4), which was greatly influenced by the molar ratio of acid to phosphate. In addition, by the extraction solution with an optimal ratio of 0.2 M HCl/0.1 M KH2PO4, As extraction efficiency differed with soil types, showing 79.6, 44.1, and 61.0% in S1, S2, and S3, respectively. The reason can be ascribed to the blocking of the available As readsorption sites by phosphate ions, the sites seemed to mainly reside on the residual amorphous Fe-oxides in soil. (C) 2015 Elsevier B.V. All rights reserved.