Technical Abstract: Plastic tarps are often applied to the soil surface following soil fumigation to reduce fumigant emissions to the atmosphere, however, these tarps are permeable to methyl bromide and other fumigants. We developed an approach to measure the mass transfer coefficient (h) of fumigant compounds across agricultural films; h being a measure of the resistance to diffusion. This information is required to develop new fumigation practice to reduce emissions. This approach uses static sealed cells, where fumigant vapor is spiked to one side of the film and the concentrations of both sides of the film are monitored until equilibrium. An analytical model, which includes sorption to and diffusion across the film membrane, is fitted to the data to obtain h. The method was used to assess the permeability of several agricultural films to fumigant compounds, and to determine the impact of temperature and condensed moisture on film permeability.