Diammonium lithium and diammonium rubidium oxyfluoro vanadates have been synthesised by solid state reactions at room temperature and their thermal decomposition mechanism determined by using simultaneous TG/DTA. The reaction intermediates have been synthesised separately and the residues identified by XRD to confirm the decomposition mechanism. In the case of the lithium compound, removal of two NH4F molecules takes place in three steps, whereas in the case of the rubidium compound this takes place in two steps.