Continuous measurements of peroxy radicals (HO2+RO2), the photolysis rate coefficients j(NO2) and j(O1D), and a number of other trace gases including O3, NOx, and CO, were carried out at the alpine site at Jungfraujoch (3580 m asl), Switzerland, in April/May 1996 during the Free Tropospheric Experiment (FREETEX '96). In spring, the Jungfraujoch Observatory is very often located in the lower free troposphere. A mean midday concentration of 17.1¿2.2 parts per trillion by volume (pptv) was calculated from continuous measurements of the sum of inorganic and organic peroxy radicals using the chemical amplification technique. The average diurnal cycles for peroxy radicals and the photolysis rate coefficient j(O1D) during FREETEX '96 showed a statistically more significant square root than first-order relationship, indicative of a relatively clean atmosphere at the mountain site, where peroxy radical self- and cross-reactions were the dominant peroxy radical loss processes. The average ozone diurnal cycle during FREETEX '96 displayed an increase of about 2 ppbv during the daytime despite the square root relationship. Similar results were derived from individual days during the campaign. A simple photochemical box model indicated that there is regime in NO concentration from about 20 pptv to 100 pptv where a better square root (than first-order) correlation between peroxy radicals and j(O1D) is accompanied by net ozone production. Taking into account that median daytime concentrations of NO at Jungfraujoch range from 51 pptv in March to 93 pptv in April, it is suggested that Jungfraujoch, during FREETEX '96, has been in the net ozone production regime in spite of the observed square root dependence of peroxy radicals on j(O1D). ¿ 1999 American Geophysical Union