Technological applications of ionic liquids (ILs) can extend over a wide range of temperatures. For several phosphonium based ionic liquids the thermophysical properties were measured such as temperatures and enthalpies of fusion and crystallization, solid and liquid heat capacities and temperatures of decomposition. A special attention was also paid on the identification of the decomposition products of ILs at high temperature. thermal data and temperatures of transition were measured using a mDSC 2920 calorimeter (TA instrument), at temperatures from -60 up to 450°C while a thermogravimetric balance from Setaram coupled to a mass spectrometer was used to determine the weight loss of an IL under a controlled atmosphere (N2 or air) along with the identification of volatile decomposition products.