Gas diffusion in soil is governed by the gas diffusion coefficient (Dp) and its variation with air-filled porosity (ε). Accurate or an upper-limit (risk assessment standpoint) prediction of Dp(ε) is essential when carrying out gas transport and fate calculations. We developed a Dp(ε) model for relatively unstructured soil separating the individual resistance of soil air, solids, and moisture to Dp. Assuming the total soil resistance to gas diffusion can be described by three power-law terms representing air-content reduction, solids-induced tortuosity, and water-induced disconnectivity yields the so-called Soil Air Phase Individual Resistances (SAPHIR) model. The SAPHIR model predicts Dp as a function of the actual ε, a particle shape factor (p), the volumetric soil water content (θ), and a water-blockage factor (w). The Dp(ε) was measured at different θ on repacked and undisturbed soil samples. The new Dp data combined with literature data implied values of p in the interval 0 to 1 and w in the interval 1 to 7, depending on particle diameter, fine-particle content, and compaction. Tested against 810 measurements of Dp on undisturbed soils, SAPHIR with average values of p = 0.6 and w = 3 performed equally well or better than traditional models; however, the test implied a need for different parameter values for more sandy soils (lower p and higher w), as well as for more compacted soils (lower p).