Osmotic coefficients of dilute aqueous solutions of ter-BuEt3NI, sec-BuEt3NI, iso-BuEt3NI, Bu2Me2NI, and Bu3EtNI are measured by the isopiestic method at 298.15 K. A branched isopiestic cell was used. The osmotic coefficients of tetraalkylammonium solutions were analyzed with the Debye–Hückel limiting law. The results show that the osmotic coefficient varies in the following way: Pr3EtNI11 > n-BuEt3NI11 > Me4NI12 > Et4NI11 > ter-BuEt3NI > sec-BuEt3NI > iso-BuEt3NI > Bu2Et2NI > Bu3EtNI. The set of Pitzer parameters b = 1.2, α1 = 2.0 was used. The results were fitted to the Pitzer model, and the parameters β0, β1, and Cγ were calculated. The results was interpreted in terms of solute–solvent interactions.