A bench-scale system was developed that through its use contributes toward a more fundamental understanding of the composting process. The computer-controlled system maintained temperature and moisture levels within narrow ranges in thin layers of compost positioned on nylon mesh trays within the vessel. This system minimized complications caused by gradients observed in earlier systems. Specific O-2 consumption and CO2 evolution rates were determined on the basis of dynamic mass balances. Water was introduced intermittently. Microbial biomass (ATP) was monitored at specific time intervals throughout the process. The system allows close observation of decomposition of feed materials at constant temperature and moisture levels throughout the process.