Dynamic TGA studies of phosphonium ionic liquids have reported thermal stabilities of 300 °C or higher for these compounds. This is often an overestimation of the real thermal stability. The chosen technique as well as the experimental parameters can influence the thermal stability. In this paper, the thermal stability of commercially available Cyphos IL 101 is studied. The effect of the nature of the atmosphere (air or inert gas), the purity of the sample, the heating rate and presence of a metal on the short-term and long-term stability of commercial Cyphos IL 101 is investigated. The thermal decomposition products are characterized using thermogravimetric analysis coupled to mass spectrometry (TGA-MS). Impurities present and higher heating rates lead to an under- and overestimation of the thermal stability, respectively. The presence of oxygen leads to a lower thermal stability. In contrast, adding metal chlorides to the ionic liquid causes an increase in the thermal stability. The chloride anions are coordinated to the metal ion, so that the Lewis basicity of the anions is reduced. Also this paper gives insights in the behavior of Cyphos IL 101 at high temperatures, which is of relevance for possible application of this ionic liquid in high-temperature industrial processes.