This study helps to establish the physical and chemical processes responsible for the generation and retention of gases within high-level waste contained in Tank 101-SY on the Hanford Site. Conducted on a laboratory scale with simulated waste, this research supports the development of mitigation/remediation strategies for Tank 101-SY. The simulated waste formulations used here are based on the composition of actual waste. Selected physical properties of the simulated waste are compared to properties of actual Tank 101-SY waste samples. There is a close correlation between the simulated waste and the actual waste from the convective slurry layer in Tank 101-SY. Gas retention in the simulated waste occurs by bubble attachment to solid particles. This attachment phenomenon is related to the organic constituents (HEDTA, EDTA, and citrate) in the simulated waste. A mechanism is discussed that relates the gas bubble/particle interactions to the partially hydrophobic surface produced on the solids by the organic constituents. Laboratory studies using aged simulated waste show that significant gas generation occurs thermally at current tank temperatures (approx.60°C). Gas compositions include those gases produced in actual tank waste, primarily N2, N2O, and H2. Gas stoichiometries are greatly influenced by several organic and inorganic constituents within the simulated waste.