A new route to fluoropurines from chloropurines is described. Quaternisation of chloropurines with trimethylamine can give either purinium chlorides or, through concomitant loss of elements of hydrogen chloride, the betaine forms. The quaternary groups in both types of derivative undergo displacement by fluoride ion, under mild conditions, producing the corresponding fluoropurines. By this means also, unprotected purine nucleosides can be successfully fluorinated.Examples of reactions of fluoropurines are cited which demonstrate the enhanced reactivity of the halogen atom in these compounds compared with the chloro-analogues.