The apparent and partial molar volumes in aqueous solution were obtained for (n-Bu)₄PBr and (n-Bu)_4-n Ph _n PCl (n=1–4) at six temperatures from 1 to 55°C. The apparent molar expansibilities were also obtained. The hydrophobic character of the cations is reduced by replacing butyl groups with phenyl groups, as evidenced by the decrease in the magnitudes of the B _v -coefficient (negative for all n) and of the temperature dependent extrema found in the apparent molar volumes and expansibilities as a function of concentration. However, the extrema exist even with BuPh₃PCl at low temperatures. The result suggests that the phenyl groups weakly affect the butyl cospheres and cation-cation interactions.