This paper reviews how isothermal microcalorimetry has been used to characterize the interaction between various lactose powders and water vapour. Microcalorimetry monitors the heat flow from processes proceeding in a sample. It has been shown that the rate of transformation of anhydrous lactose to the monohydrate depends strongly upon the content of anhydrous alpha- and beta-lactose in the sample, in addition to the relative humidity. It has been further shown that microcalorimetry is an excellent technique for measuring small amounts of amorphous lactose. This was done by calculating the heat when the amorphous parts crystallized under humid conditions. Results obtained by differential scanning calorimetry are also discussed. Furthermore, references from other studies, in which lactose powders have been investigated by thermal analysis techniques, are given.