In this work the solubility and the diffusivity of argon in water and in aqueous solutions of tetrabutyl ammonium iodide, at concentrations in the range 0.02 m to 0.04 m, were measured. Measurements were made at two temperatures: 288.15 K and 298.15 K. The design, construction and the calibration of a gas-liquid contact equipment is described. It uses auxiliary equipment for the measurement of pressure and the control of temperature. This apparatus allows the measurement of diffusivity and of solubility in diaphragm cells by the pressure decline method. The measured values of the properties confirm the hydrophobic character of the [Bu]4N + cation.