A method is proposed to measure the diffusivity of vapors and gases through porous media such as soil. The apparatus consists mainly of a semicircular tube, which is closed at one end. A formula to calculate the diffusivity from the wt. losses of this tube is given. It is based on a theoretical treatment of factors which had been previously neglected. These factors are the existence of a gas current to the open end of the tube out of which the vapor diffuses and the friction incurred by such a current as it flows through a porous medium. It is shown that the latter effect may be safely ignored. The exptl. results obtained with dry quartz and soil mixtures show that the ratio between the diffusivity in a porous medium and in gaseous phase is about 0.6. Previous measurements had placed its value at 0.66.