The temporal evolution of activity profiles of the radon isotope 222Rn in the top 50 cm of soil is used to quantify diffusive gas transport in the air-filled pore space. Air is continuously collected from gas-permeable, hydrophobic membrane tubes placed at various depths under the surface and circulated through a high-sensitivity Rn detector. With a micro-processor controlled inlet system an automatic monitoring of several level is possible for extended periods of time with a typical time resolution of one hour. In addition, a new dynamic approach to measure the in-situ soil diffusivity Ds (m2/s) around the subsurface tubes is presented where the recovery from air injection is evaluated every 3 hours. Within a test period of 3 weeks Ds dropped from 10-6 m2/s to 3.10-7 m2/s at a depth of z=50 cm due to increasing water content of the soil. ¿ 2000 American Geophysical Union