The ionic liquids 1-ethyl-3-methylimidazolium acetate, [EMIM]Ac, and 1-butyl-3-methylimidazolium acetate, [BMIM]Ac, were characterized using TGA, FTIR, and UV-vis spectroscopy. Their mixtures with the protic solvents ethanol, methanol, and water were characterized with FTIR and UV-vis spectroscopy. Absorbance versus concentration calibration curves over the full concentration range from 0 to 100 wt % IL content were generated for the mixtures. The thermal stability of the ionic liquids was evaluated by dynamic TGA runs through the assessment of the onset of decomposition and peak decomposition temperatures. Additionally, isothermal TGA runs were performed at 100, 120, and 150 degrees C over a period of 24 h. The results show that these ionic liquids are not stable over long time exposures, displaying significant weight loss over time above 100 degrees C. The thermal stability of mixtures of these ILs with ethanol, methanol, water and an aprotic solvent, DMF, were assessed further with time-lapsed UV-vis spectroscopy using a high-pressure view-cell at temperatures in the range 100-150 degrees C at 35 and 350 bar over a 24 h exposure time. The UV spectra for the ethanol and methanol mixtures shifted over time at 120 and 150 degrees C. The mixtures with water did not demonstrate significant changes at any of these temperatures. This was in contrast to mixtures with DMF, in which spectral shifts occurred almost immediately at all studied temperatures. These ionic liquids and their mixtures in these solvents are important in biomass processing and their thermal stability under process conditions must therefore be given due consideration in rational process design.