The relationship between air permeability, dry bulk density and water content of porous media was investigated. Air permeability was measured using sieved, repacked 100 cm³ samples adjusted to specific water contents and dry bulk densities. For each porous medium combinations of dry bulk density and water content were selected so as to cover the entire possible range for the medium. The porous media investigated included organic and inorganic soils, composts made from sewage sludge and yard waste, and selected organic wastes. Air permeability generally varied over 2 – 4 decades for the materials investigated and decreased with increasing dry bulk density. In many cases the decrease exhibited a linear relationship between Log(air permeability) and dry bulk density, indicating an exponential relationship between the two parameters. The relationship between air permeability and water content was somewhat more complicated. At low and high water contents air permeability generally decreased with increasing water content. At intermediate water contents, however, some of the investigated materials showed increasing air permeability with increasing water content. This tendency was especially pronounced for low values of dry bulk density. An explanation for this behavior is that the materials tend to aggregate at higher water contents which creates large inter-aggregate pores which in turn leads to higher air permeability. At intermediate water contents the aggregates are stable enough to be packed especially at relatively low dry bulk densities without being destroyed, thereby creating a system of large inter-aggregate pores.