A transient method for field measurement of gas diffusion into soil is presented. The theory of the method and the results of preliminary tests are given. Diffusion takes place from a reservoir mounted on a tube driven into the soil. The theoretical analysis is used in the design of the experimental method, the relative importance of the various parameters involved in apparatus design and in measurement being evaluated. The apparatus is described and sample calculations of parameters given. When porosity is uniform with depth, diffusion coefficients obtained in the field agree well with results of diffusion measurements of other workers made under better controlled (laboratory) conditions.