The objective of this study was to determine how the key features of bioreactor landfills; increased temperature, moisture and microbial activity, affect the biological stability of the landfill material. In the first part of the study the solubilization and degradation of lignin in paper exposed to these bioreactor landfill conditions are explored. The solubility of the lignin in paper was observed at different temperatures and over 27 weeks at 55°C and the anaerobic bioconversion of office paper, cardboard and Kraft lignin was observed in bench-scale reactors over 8 weeks. As the temperature rose, lignin solubility increased exponentially. With extended thermal treatment, the dissolution of lignin continues at a constant rate. This rate increases 15 times for paper and 1.5 times for cardboard in the presence of rumen inoculum compared to un-inoculated systems. At around 6 weeks the inter-monomeric linkages between the solubilized lignin molecules began breaking down, releasing monomers. In cardboard and Kraft lignin, a significant amount of the monomers mineralize to CO₂ and CH₄ during this time period. The results indicate that small, but significant rates of lignin solubilization and anaerobic lignin degradation are likely to occur in bioreactor landfills due to both higher temperature and microbial activity. In the second part of the study, field data from the Outer Loop Recycling and Disposal Facility in Louisville, Kentucky was evaluated to determine the effectiveness of an anaerobic-aerobic landfill bioreactor (AALB) vs. the control landfill that is managed as a traditional landfill. Moisture, temperature, elevation and the amount of time the MSW has spent in the landfills (age) were measured and compared to determine the factors that affect the biological stability of the landfill. The results showed that the MSW in the AALB is more biologically stable than the MSW in the control landfill, indicating that they are more degraded. Additionally, elevation or location of the MSW was the key factor in determining the extent of MSW stability within the AALB and temperature is the key factor in determining the biological stability of the MSW in the control landfill. Higher temperatures correlated with a more biologically stable waste. The cellulose to lignin ratio (C/L ratio) and biochemical methane potential (BMP) were the main biological stability parameters used. / Master of Science