The chemistry of the stratosphere is dominated by the processes that remove the ‘odd oxygen' (atomic oxygen and ozone) generated by the photolysis of molecular oxygen. In recent years it has been established that the Chapm an reaction, O+O 3 = O 2 + O 2, cannot account for all the observed destruction of odd oxygen. Catalytic cycles involving nitrogen oxides (NO, NO 2 ), hydrogen radicals (H, HO , HO 2 ) and chlorine species (Cl, CIO) have been shown to be im portant and it is now realized that these catalytic cycles are closely linked to each other. Measurements of these trace species, their precursors (e.g. N 2 O , H 2 O , CH 4 ) and their reaction products (e.g. HNO 3 , HCL) in the stratosphere are essential to understanding its chemistry. The natural variability of the stratosphere places a premium on simultaneous measurements of those trace species that are interconverted by rapid chemical reactions.