Vapor pressures of aqueous t-butyl alcohol (TBA) were determined at 25 degrees C by the static method in the mole fraction range, 0.000 068 < x(TBA) < 0.006. The partial pressures of TBA, p(TBA), and those of H2O, p(W), were calculated by a numerical integration method developed by Boissonnas. It was pointed out that this method inevitably introduces errors in the first few data points from the dilute end, which will be self-corrected after several data points. Together with the data obtained for the range x(TBA) > 0.002 in the previous work (J. Phys. Chem. 1990, 94, 7700), reliable values of the excess partial molar Gibbs function of TBA, G(m)(E)(TBA), were obtained. It was concluded that at the most dilute point measured, x(TBA) 0.000 068 (m(TBA) = 0.0038 mol/kg), the solution is not yet dilute enough for Henry’s law to be obeyed.