The main soil-gas transport parameters, gas diffusivity and air permeability, and their variations with soil type and air-filled porosity play a key role in soil-gas emission problems including volatilization of toxic chemicals at polluted sites and the production and emission of greenhouse gases. Only limited information on soil-gas transport parameters across the vadose zone is available, especially for soil layers below the root zone. In a series of studies, we developed new data for the soil-gas transport parameters in different soil profiles and tested existing and new predictive models. In this first study, we measured gas diffusivity at different soil-water matric potentials on undisturbed soil samples for three lysimeter soil profiles down to 1.4-m depth and for two field soil profiles down to 5.6-m depth, representing a total of 22 different soil layers with soil texture ranging from sand to sandy clay loam. Five commonly used predictive gas diffusivity models were tested. The three-porosity model (TPM) performed best for both shallow and deep soil layers. The tortuosity–connectivity parameter X in the TPM varied with soil texture and pore size distribution, and the TPM predicted well the depth distributions of measured soil-gas diffusivities. The TPM also requires less detailed information on the soil-water characteristic curve than other well-performing predictive models, and is therefore recommended for predicting variations in soil-gas diffusivity within the vadose zone.