Experiments were conducted with simulated Municipal Solid Waste (MSW) to understand the impact of pressure, moisture, and temperature on MSW decomposition under simulated landfill conditions. Three experimental phases were completed, where the first two phases provided baseline results and assisted in fine tuning parameters such as pressure, temperature, gas composition, and moisture content for phase three. The manuscript focuses on the results from third phase. In the third phase, the composition of the gases evolved from representative MSW samples was tested over time in two pressure conditions, 101 kilopascals (kPa) (atmospheric pressure) and 483 kPa, with varying moisture contents (38 to 55 wt%) and controlled temperatures (50 to 200 °C) in the presence of biological inhibitors. The headspace in the reactor in phase three was pressurized with gas mixture of 50/50 (vol%) of methane (CH4) and carbon dioxide (CO2) setting the initial CH4/CO2 gas composition ratio to 1.0 at time t = 0 days. The results established moisture ranges that affect hydrogen (H2) production and the CH4/CO2 ratio at different temperature and pressure conditions. Results show that at 85 °C, there was a change in the CH4/CO2 ratio from 1.0 to 0.3. Additionally, moisture contents from 47 to 43.5 wt% caused the CH4/CO2 ratio to increase from 1.0 to 1.2, yet from 43.5 to 38 wt%, the ratio reversed and declined to 0.3, returning to 1.0 for moisture levels below 38 wt%. Thus, moisture levels above 47 wt% and below 38 wt%, for the system tested, allow thermal reactions to proceed without a measured change in CH4/CO2 ratio. H2 generation rates follow a similar trend with moisture, yet definitively increase with increased pressure from 101 kPa to 483 kPa. The observed change in solid MSW and gas composition under controlled pressure, moisture, and temperature suggests the presence of thermal reactions in the absence of oxygen.