This paper presents the measurements of OH and HO2 radical concentrations as well as photolysis frequencies of different molecules during the Berliner Ozone (BERLIOZ) field experiment in July/August 1998 at the rural site Pabstthum about 50 km NW of Berlin. Radical concentrations were measured using laser‐induced fluorescence (LIF) spectroscopy, while filter radiometers and a scanning spectroradiometer were used to obtain photolysis frequencies. The radical data set covers the time period from 20 July to 6 August and consists of more than 6000 simultaneous measurements of OH and HO2 with a typical time resolution of about 90 s. The maximum OH and HO2 daytime concentrations were 8 × 106 and 8 × 108 cm−3, respectively. While nighttime values of OH were usually below the detection limit of our instrument (3.5 × 105 cm−3), HO2 did show significant concentrations throughout most of the nights (on average 3 × 107 cm−3). The OH concentration was mainly controlled by solar UV radiation and showed a high linear correlation with J(O1D). A deviation from this general behavior was observed around dawn and dusk, when OH concentrations well above the detection limit were observed, although J(O1D) was essentially zero. A comparison with data sets from previous campaigns revealed that even though the linear correlation is found in other environments as well the slope OH/J(O1D) differs significantly. The diurnal cycles of HO2 were less dependent on the solar actinic flux but were predominantly influenced by NO. During episodes of high NO, HO2 remained below the detection limit (1 × 107 cm−3) but started to rise rapidly as soon as NO started to decrease.