Radon-222 diffusion length has been measured for 28 Danish soils and sediments. Samples were placed in a diffusion column connected to a 222Rn reservoir and the transport of 222Rn through the column was measured. The diffusion length was found by comparing the measured data to time-dependent diffusion theory. Dry silt, sand and gravel materials showed diffusion lengths from 160 cm to 190 cm. For wet clay materials, diffusion lengths of about 1 cm were found.