Continuous measurements of peroxy radicals were carried out using the chemical amplification technique during three intensive FREE Tropospheric EXperiments, (FREETEX) in 1996, 1998 and 2001 at the Jungfraujoch (3,580 m asl) in the Swiss Alps, in combination covering the period of ozone build-up from late winter to late spring. The monthly medians of the peroxy radical measurements from 10:00 to 13:00 UT in February, March, April, and May, after filtering out the more cloudy conditions, show a clear seasonal behaviour with a gradual increase of the peroxy radical mixing ratios from around 12 pptv in February to around 20 pptv in May. This data represents among the first semi-continuous seasonal observations of peroxy radical concentrations, which indicates the important and possibly dominant role of in-situ photochemistry in the observed ozone build up from winter to spring in the lower free-troposphere over the Alps.