Rarely does the atmosphere allow direct observation of the photochemical evolution of ozone. In most of the troposphere and lower stratosphere this slow chemistry cannot be understood without including much larger changes caused by the circulation. Yet in the tropical stratosphere, where ozone-poor air of tropospheric origin enters and rises slowly in near isolation, it can be demonstrated that O3 is created by dissociation of O2 at a rate consistent with current theory. The parallel photolytic destruction of the unreactive source gases (for example, N2O and CFCl3) and the consequent evolution of chemically active odd-nitrogen (NOy) and chlorine (Cly) species, however, indicate a small amount of mixing of much older, photochemically aged air from the midlatitude stratosphere into this tropical plume. ¿ American Geophysical Union 1996