Air permeability was measured as a function of gravimetric water content and dry bulk density for six sieved, repacked porous materials with organic matter contents ranging from 0 to 88%. The six media used were two relatively fine sandy soils, a filter sand, a peat soil, and two commercial composts made from sewage sludge and yard waste, respectively. The materials were initially wetted from air dry to the desired water content, after which measurements were done. Selected combinations of material gravimetric water content and dry bulk density were used in the permeability measurements. The combinations were selected to cover the entire possible ranges of water content and dry bulk density for each material. The six materials exhibited very different relationships between air permeability, water content, and dry bulk density, exhibiting both increasing and decreasing air permeability with water content depending on their structural behavior. All six materials had regions in their water content–dry bulk density domains where air permeability tended to increase with increasing water content, indicative of increasing structure formation in these regions. This tendency was strongest for the two sandy soils and weakest for the filter sand, which exhibited almost no structure formation. Increased structure, as indicated by increasing air permeability with increasing water content, was strongest for low values of dry bulk density in all six materials.