Peroxides are produced as termination products in atmospheric chain reactions involving peroxy radicals, both organic and inorganic. They are the principal sink for radicals produced in the troposphere from the photolysis of ozone in the presence of water vapour and as such are excellent indicators of the extent of free radical chemistry taking place at any given location. Their measurement is relatively simple and data on the concentration of peroxides in the atmosphere with respect to time and space can be collected easily and extensively. New data on peroxide measurements collected at different parts of the atmosphere, principally by the Meteorological Office C-130 Hercules aircraft are presented. They indicate that the extent of hydroxyl radical chemistry during the summer is controlled mostly by the water vapour content of the atmosphere. Both negative and positive correlations are observed between ozone and peroxide concentrations in vertical profiles over the North Atlantic Ocean and the equatorial Pacific. The negative correlations demonstrate that the ozone concentration throughout the troposphere is determined mostly by in situ photochemistry. This is borne out by the close correlation between calculated and measured concentrations of peroxides in vertical profiles. Positive correlations over the North Atlantic allow us to make an estimate of the amount of ozone, present there in the summer, which is formed from tropospheric as opposed to stratospheric chemistry.