Standard Gibbs energies of transfer of potassium tetraphenylborate from water to (water + t-butyl alcohol) (TBA) mixtures at T = 298.15 K have been studied. The mole fractions of TEA in the mixed solvent are 0.00, 0.01, 0.02, 0.03, 0.04, 0.045, 0.05, 0.06, 0.07, 0.08, 0.09, 0.10, 0.20, 0.40, 0.60, 0.80, and 1.00, respectively. The primary medium effect of potassium tetraphenylborate in the mixed solvent is discussed.