The osmotic coefficients of Bu 4NBr, sec-Bu 4NBr, iso-Bu 4NBr, Bu 2Et 2NBr and Bu 3EtNBr 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–Hückel 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.