In the stratosphere, dissociation of H2O, CH4 and H2 is brought about, mainly by reaction processes with excited oxygen atoms produced by the photodissociation of ozone. A discrepancy noted between theoretical and observational concentrations of O3 in the upper stratosphere suggests two possible explanations : solar radiation fluxes for O2 photodissociation which are too large along with too large rate coefficients and absorption cross sections or ozone reduction brought about by the effect of hydrogen compounds or of nitrogen oxides. The reaction of the excited oxygen atom with methane and nitrous oxide leads to a destruction of these two molecules in the stratosphere which corresponds to a production of carbon monoxide with water vapor and of nitric oxide, respectively. The vertical distribution of water vapor is not affected by its dissociation in the stratosphere since its reformation is rapid. The fact that the ratio of the hydroxyl and hydroperoxyl radical concentrations cannot be determined with adequate precision complicates the calculation of the destruction of ozone which occurs through reactions of OH and HO2 not only with atomic oxygen in the upper stratosphere but also with CO and NO in the lower stratosphere, respectively. The same difficulty arises in connection with the dissociation of nitric acid molecules formed by the reaction of OH and NO2; destruction processes by photodissociation or by reaction with OH are not yet known with precision. Another difficulty, of a different kind, is that the nitric oxide concentration is not well known at the stratopause level.