Isothermal microcalorimetry gas flow vapour sorption experiments can be used to assess the surface energetics of crystals, the formation and/or loss of hydrates as a function of humidity and the onset of deliquescence, and to identify the presence of amorphous content in powders. These applications require different aspects of calibration. For gas flow instruments, it is simple to calibrate the humidity in the measuring cell by use of a saturated salt solution. To allow the power output to be compared between laboratories, there is a need for a standard test reaction for adsorption behaviour. The use of sealed ampoules containing reservoirs of saturated salt solutions has provided a means by which physical transformations (such as crystallisation) in powders can be followed. The study of crystallisation of amorphous samples involves a complex mixture of exotherms and endotherms which make it difficult to standardise the exact conditions which are used. Comparison of data between different laboratories can therefore become difficult. It is difficult to think of a calibration standard for these type of experiments, as the sample is usually thermodynamically unstable (a physical change is induced during the experiment). The use of water sorption in isothermal microcalorimeters is growing in popularity as a means of characterising both adsorption to and absorption into powders, but there are complex calibration issues that still need to be addressed,