In order to investigate the adsorptive properties for As(III) and As(V) at 303 K, the magnetite was prepared by adding the aqueous sodium hydroxide solution to the mixture solution of Fe2+:Fe3+ = 1:2 using iron chloride. The adsorption of As(III) and As(V) was dependent on equilibrium pH and showed a maximum value at pH 6 and 5, respectively. From their chemical species existing at each pH and the pH of the zero point of charge of the magnetite surface, As(V) was probably adsorbed on the surface of the magnetite through the electrostatic force, while the adsorption of As(III) was different from that of As(V). It is probable that As(III) was absorbed through an ester reaction on the magnetite. The adsorption capacities of As(III) and As(V) were 2.77 x 10(-1) mmol(.)g(-1) and 2.28 x 10(-1) mmol(.)g(-1), respectively. The adsorption equilibrium constants for As(III) and As(V) were 4.74 d m(3.)mmol(-1) and 12.1 d m(3.)mmol(-1), respectively, suggesting that the magnetite has higher affinity to As(V). An aqueous sodium hydroxide solution was useful as an eluent for desorption of As(III) and As(V).