An experimental study was conducted to assess the relative contributions of molecular and Knudsen diffusions for gas-phase transport in unsaturated silica flour with an intrinsic permeability of 1 ∼ 2 × 10−14 m2. Single-gas phase flow and binary diffusion experiments were performed on the same soils to determine the Knudsen diffusion and the effective gas-phase diffusion coefficients, respectively. These results were further used to assess the diffusibility factor for Fick's law of diffusion and the obstruction factor for the Dusty Gas Model theory. These factors were used to account for impedance to gas diffusion caused by the tortuous nature of soil pores. The Knudsen diffusion coefficient was found to be one to two orders of magnitude greater than the effective diffusion coefficient for silica flour at water saturations ranging from 53 to 75% (v/v), which suggested that the Knudsen effect has negligible impact on the effective diffusion coefficient for the unsaturated silica-flour system at water saturations lower than 75%. For unsaturated silica flour with air-filled porosity <0.2, the diffusibility factor (or the obstruction factor) value can be more than 10 times smaller than the values estimated by other diffusibility correlations published in the literature.