Abstract The adsorption of glycine, Dl-serine, l-threonine, l-asparagine, l-glutamine, l-valine, l-leucine, l-methionine, and l-tryptophan at the mercury-aqueous sodium fluoride solution interface has been studied by measuring differential capacities, potentials of zero charge, pzc, and surface tensions at the pzc. The standard adsorption free energies for these amino acids and for the amino acids previously studied have been used to deduce hydrophobicity scales of amino acid side chains at positively charged, negatively charged, and uncharged interfaces. The hydrophobicity scale at the uncharged interface was basically similar to that of Nozaki and Tanford. When the surface was charged positively, the positions of serine and alanine, and of threonine and α-aminobutyric acid, in the scale were reversed. This reversion was primarily ascribed to the difference in the positions of maximum adsorption potentials between the amino acids with an aliphatic side chain and those with a hydroxyl group in the side chain.