Impedance titration was used to determine the surface dissociation characteristics of short-chain carboxyl mercaptan self-assembled monolayers (SAMs). Based on the change of the cyclic voltammetric peak current and the charge-transfer resistance,which was related to pH value of the solution,the surface pKα of mercaptoacetic acid(MA),3-mercaptopropionic acid(MPA) and ω-mercaptohexanic acid(MHA) self-assembled membranes,with ionic strength being 0.1 mol/L,were determined to be 5.20,4.80,7.40,respectively. In addition,factors such as time needed for assembling,structure of monolayers and ionic strength,which effected the surface pKα,were studied as well. Such surface pKα shifts were sufficiently explained by interactions between interfacial molecules and hydrophobicity. Impedance titration was used to determine the surface dissociation characteristics of short-chain carboxyl mercaptan self-assembled monolayers (SAMs). Based on the change of the cyclic voltammetric peak current and the charge-transfer resistance, which was related to pH value of the solution , the surface pKα of mercaptoacetic acid (MA), 3-mercaptopropionic acid (MPA) and ω-mercaptohexanic acid (MHA) self-assembled membranes, with ionic strength being 0.1 mol / L, were determined to be 5.20, 4.80, 7.40, respectively. Factors such as time needed for assembling, structure of monolayers and ionic strength, which effected the surface pKα, were studied as well. Such surface pKα shifts were stimulated interactions by interfacial molecules and hydrophobicity.