Density measurements have been made on aqueous solutions of t-BuEt₃NI, s-BuEt₃NI, i-BuEt₃NI, Bu₄NI and Et₄NI at 298,15 K from 0,01 to 0,05 m. The apparent molal volumes V0₂ calculated from the V _2,ϕ (m) plots for the salts show a decrease value: n-BuEt₃NI > i-BuEt₃NI > s-BuEt₃NI > t-BuEt₃NI. The data analysis may suggest that the cations with the butyl isomers perform two groups: n-BuEt₃N⁺ and i-BuEt₃N⁺ with the highest V ₂ ⁰, and t-BuEt₃N⁺ and s-BuEt₃N⁺ similar to Et₄N⁺ one. The peculiarities of the experimental observation may be interpreted as a consequence of the structural effect over the intermolecular forces that affect the solute-water interactions deeply.