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Samples of 1-2 mm crumbs from a clay loam under permanent pasture were equilibrated at -5 kPa water potential then compacted to varying degrees. Gas diffusion coefficients D, (hydrogen through air), were measured immediately on compaction, again after re-equilibration at -5 kPa, then at other water contents between saturation and dryness. The relationship between diffusion coefficient and air content, E , was, as elsewhere, in two parts (dD/dE small for drainage of pores within crumbs; large for pores between crumbs), but the transition from one part to the other occurred at smaller air contents with increased compaction. The air content at which D approached zero as the samples wetted was greatest in the loosest soil. Compaction from a bulk density of 0.86-1.29 g ~m -~ decreased the relative diffusion coefficient, DIDO (DO is the diffusion coefficient without impedance), from 0.35 to 0.22 (by 38%) at complete dryness, but from 0.19 to 0.035 (by 82%) in the soil initially at -5 kPa.On re-wetting and re-equilibrating at -5 kPa, DIDO decreased further to 0.008 (total 97%) because of extra water held in the now smaller pores of the compacted soil.No single relationship between D/Do and E fitted the results for even this one soil. CALL, F. 1957. Soil fumigation. V. Diffusion of ethylene dibromide through soils. Journal ofthe Science ofFood and Agriculture3,143-150. CURRIE, J.A. 1 9 6 0~. Gaseous diffusion in porous media. Part 1. A non-steady state method. British JournalofAppliedPhysics 11,314-311. CURRIE, J.A. 19606. Gaseous diffusion in porous media. Part 2. Dry granular materials. British Journal ofApplied Physics 11,3 18-324. CURRIE. J.A. 1961. Gaseous diffusion in porous media. Part 3. Wet granular materials. British Journal ofdpplied Physics 12,275-281.