Endothermic decomposition reactions (solid 1 --> solid 2 + gas) generally affect only part of the inter-atomic bonds of the solid 1 structure. In consequence, the morphology and external dimensions of particles remain unchanged (the particles of solid 2 are referred to as pseudomorphs of those of solid 1). The gas release normally leads to a decrease of the molar volume of the precursor solid, resulting in cracking and formation of intra-particle porosity, This work deals with such a reaction: the dehydration of gypsum into sub-hydrated and anhydrous phases. Ultrathin (010) gypsum cleaved plates have been dehydrated under controlled conditions and studied by electron microdiffraction, All reactions have been shown to preserve four different sets of three-dimensional orientation relations between all structures. These topotactic orientation relations allow us to clarify the associated atomic rearrangements. The reactions are described in a simplified net common to all structures, which allows to predict the microstructure of the resulting pseudomorphs.