Knudsen effusion mass spectrometry (KEMS) has long been a key technique to obtain thermodynamic data based on vapor pressures. A Knudsen cell is small enclosure (1 cm diameter x 1 cm long) with a well-defined orifice (typically 1 mm diameter). This allows equilibrium or near equilibrium to be attained in the cell and the ensuing vapour to be sampled. Mass spectrometry is an ideal tool for probing the generated vapour. It given the composition and can be related to the absolute vapour pressure. The temperature dependence of this vapour pressure yields heats of vaporization, which in turn can be related to heats of formation for pure compounds.