SUMMARYRelative gas diffusivity(D/D0)was measured in peat cores equilibrated to set moisture tensions between zero and ‐ l0 kPa, using the radioactive gas85Kr. A relationship between relative diffusivity and the air filled porosity was obtained, which showed lower values ofD/D0at air‐filled porosities above about 0.13m3m−3, and higher values at porosities below about 0.10m3m−3than some of those found in the literature for mineral soils. The likely effects of shrinkage behaviour on drying in the field, on the relationship betweenD/D0and air‐filled porosity, are