In this work, the experimental viscosities of concentrated aqueous glycine, L-alanine and glycylglycine in tetra-n-alkylammonium bromides (R4NBr with R = CH3, C2H5, C4H9) are reported. The viscosities of aqueous amino acids/peptide are enhanced in the presence of R4NBr. These enhanced viscosities are discussed in terms of hydrophobic hydration and structure-making and -breaking abilities of the components involved. The mixing effects due to the interaction between amino acid/peptide and R4NBr are understood in terms of Gibbs free energy of activation. A simple equation is proposed to accurately analyze the effect of the above ions on the viscosity of the amino acids and peptide. (C) 2004 Elsevier Ltd. All rights reserved.