The effect of tetra-n-alkylammonium bromides, R4NBr (R = CH3, C2H5, C4H9) on the densities, rho, of glycine, L-alanine and glycylglycine are reported at T = 298.15 K. The apparent molar volumes of amino acids in aqueous tetra-n-alkylammonium salts, phi(VAJW), and of tetra-n-alkylammonium bromides in aqueous amino acids and peptide, phi(VJAW), are calculated from the measured densities. Both phi(VAJW) and phi(VJAW) have been analysed accurately using a simple equation. Positive transfer volumes are observed for glycine, L-alanine and glycylglycine in the presence of R4NBr. Tetra-n-butylammonium bromide shows almost double increase in the transfer volumes of amino acids or peptide than tetramethyl- or tetraethylammonium bromides. Negative transfer volumes for the tetra-n-alkylammonium bromide salts are noted in aqueous amino acids or peptide due to large tetra-n-alkylammonium cation undergoing hydrophobic hydration. (C) 2004 Elsevier Ltd. All rights reserved.