Bulk diffusion and gaseous permeability coefficients were measured in situ in most morphologic horizons of four soil profiles in central Pennsylvania. Such data are rare in the literature. From the eluvial to the illuvial horizons of individual soil profiles, bulk diffusion coefficients generally decrease by nearly an order of magnitude, and gaseous permeability coefficients decrease by about two orders of magnitude. In all four profiles, the diffusion and permeability coefficients are higher in the upper, coarsely textured horizons than the lower horizons and, at corresponding depths, in more coarsely textured than finely textured pedons.The accuracy of the diffusion coefficients is confirmed by the similarity of an observed radon-222 (Rn) concentration profile to that estimated using the measured diffusion coefficients and a two-dimensional finite difference model.Several published methods of estimating bulk diffusion coefficient from air-filled porosity are statistically compared with the data following log transformation. Although all the methods tested were highly correlated to the in situ data, the estimates of Millington (1959, Science, 130:100–102) and Sallam et al. (1984, Soil Science Society of America Journal, 48:3–6) produced values most similar to those measured.Logarithmically transformed values of the bulk diffusion and permeability coefficients are highly correlated with each other for both our data and previously published data. This relationship holds true for eight different soils measured by separate researchers using different methodologies and appears to be generally applicable. Since the permeability values encompass a larger relative range than the diffusion values, measured gaseous permeability coefficients can be used to estimate bulk diffusion coefficients. This empirical approach to estimating diffusion coefficients is useful in that, in addition to air-filled porosity, permeability reflects the continuity and tortuosity of the air-filled pore system. Since the continuity and tortuosity of air-filled pores affect the diffusion coefficient, and neither of these properties are directly reflected in estimates based on air-filled porosity, estimation of diffusion coefficient using permeability accounts for important soil properties not directly accounted for with methods based on air-filled porosity alone.