This report summarizes progress to date on the thermal stability of imidazolium salts being considered for application as heat transfer and thermal storage fluids in solar parabolic trough power systems. Imidazolium salts are a subset of the general class of molten salts. They are termed ionic liquids because many have freezing points at or below room temperature. This class of salts was selected for initial study because there were many examples that were reported to be stable at high temperatures. These reports were usually based on the results of standard thermal gravimetric analysis (TGA) methods. Work by our subcontractor at the University of Alabama and at NREL showed that slow heating rates or when the temperature is held constant for long times resulted in decomposition temperatures that are much lower than those found with the usual TGA methods. We have used a TGA technique that allows calculation of the rates of thermal decomposition as a function of temperature. The results lead us to the conclusion that the imidazolium salts known to be the most thermally stable would not have useful lifetimes above about . At present this determination is based on the rough approximation that the fluid in a solar trough system experiences a constant, high temperature. Better estimates of the useful lifetime will require a system model that takes into account the time at temperature distribution of a fluid moving through the different components in a solar plant.