Atomistic characterisation of phase transitions in organic materials is important because of the role played by polymorphism in pharmaceuticals and agochemicals. Conventional crystallographic experiments yield an average view of the crystalline state, a substantial approximation in the context of dynamic phenomena like phase transitions. For example there is no information in an ¿average¿ crystal structure on defects which play a crucial role in reconstructive phase transitions. We will study an unusual transition in pyridine which is only observed if the sample is deuterated. Pyridine-d5 forms a complex Z¿=4 phase on cooling from the liquid. This phase can be cooled to 2K, where it is metastable. On warming to 170 K it transforms to a Z¿=1 phase, which transforms back to the Z¿=4 phase at 215 K. Total scattering analysis will explore the role of stacking defects in these transitions.