The osmotic coefficients of Bu4NBr, sec-Bu4NBr, iso-Bu4NBr, Bu2Et2NBr and Bu3EtNBr have been measured by isopiestic method at 293.15 K in dilute aqueous solutions. A branched isopiestic cell was used. The osmotic coefficients of tetraalkylammonium solutions were analyzed comparing these with the Debye-Huckel limiting law. The order of the osmotic coefficient variation is used for qualitative deduction of the solute-solvent interactions occurring in these solutions. The osmotic coefficients have been correlated using the ion interaction model of Pitzer. The results show that the Pitzer model successfully correlate the osmotic coefficients. (c) 2006 Elsevier B.V. All rights reserved.