An experimental apparatus was constructed to measure the structural parameters of organic porous media, i.,e. mechanical strength, air-filled porosity, air permeability, and the Ergun particle size. These parameters are critical to the engineering of aerobic bioconversion systems and were measured for a straw-manure mixture before and after 13 days of in-vessel composting. Porosity was measured using air pycnometry at four (day 0) and five (day 13) moisture levels, with each moisture level tested at a range of different densities. Tested wet bulk densities varied with moisture level, but dry bulk densities generally ranged from 100 to 200 kg m(-3). At each moisture/density combination, pressure drop was measured at airflow rates ranging from 0.001 to 0.05 m sec(-1), representing the range of airflow rates found in both intensive and extensive composting. Measured air-filled porosities were accurately predicted from measurements of bulk density, moisture, and organic matter content. Reductions in air-filled porosity at increasing moisture content were accompanied by an increase in permeability, apparently due to aggregations of fines. This aggregation was quantified by calculating an effective particle size from the Ergun permeability relationship, which increased from 0.0002 m at 50% moisture to 0.0021 m at 79% moisture. The range of airflow velocities reported in composting systems requires consideration of the second-order drag force term, particularly at velocities approaching 0.05 in s(-1) for the higher moisture treatments tested. Calculated permeabilities for the matrix ranged from 10(-10) to 10(-7) m(2), varying with both air-filled porosity and moisture. Mechanical strength characterization provided a means to predict the effects of compaction on air-filled porosity and permeability of porous media beds. The results of this investigation extend porous media theory to the organic matrices common in solid-state fermentations and help build a framework for quantitative and mechanistic engineering design.