Scenario-specific numerical simulations were performed in order to assess the diffusive spreading of volatile fuel constituents and their biodegradation in the unsaturated soil zone. This is especially important in arid regions where the groundwater table is far below the surface and groundwater recharge is limited. The model results compare well with data from a controlled outdoor test with an emplaced source of kerosene. Modelling results illustrate that the overall biodegradation rates depend mainly on properties of the fuel constituents such as Henry's Law constant, on the individual degradation rate constants, the soil water content and temperature. The contaminant transfer rates into groundwater can be determined provided the spreading processes in the unsaturated zone are understood.