Determination of the effective gas-phase diffusion coefficient is important to quantify gas diffusion for a number of environmental pollution problems involving migration and release of hazardous gases or organic vapors in soils. Conventionally, the only considered transport mechanism for this coefficient is molecular diffusion. This study additionally considers the effects of Knudsen diffusion and nonequimolar flux on this coefficient and proposes a relationship of this coefficient with both molecular and Knudsen diffusion coefficients, ratio of molar masses and composition of component gases for gas diffusion in soils. Through investigating calculation results of this relationship for hypothetical diffusion systems having different soil permeabilities and diffusing gases at a range of water saturation levels, it is found that the Knudsen and nonequimolar effects can only be substantial for systems with dry soil permeabilities less than 10 m at any saturation. The Knudsen effect on this relationship is further demonstrated through comparing results obtained from this relationship to the measured effective diffusion coefficients for 72 soil cores, which all have high silt and clay contents, given in the literature.