Abstract: The adsorption amounts of decylmethylsulfoxide (DEMS) at aqueous solution/carbon black interface and the contact angles of DEMS aqueous solutions on graphite plate have been measured. The influences of temperature, salt (NaCl) and acid(HCl) have been investigated. Typical two plateaux (LS) type adsorption isotherms have been shown. The adsorption ammounts at the first plateau are about 5.7—6.4 μmol·m~(-2) and those at the second one are 42—48 μmol·m~(-2). As the adsorption increasing, the wettability and suspensibility of carbon black in aqueous solution of DEMS were improved obviously. Applying the two-step model of surfactant adsorption on solid/liquid in terface and the general isotherm equation for adsorption of surfactant at solid/liquid in terface (eqn(1))~[5], the experimental results have been interpreted qualitatively and quantitatively.