Beside isoprene, monoterpenes are the non-methane volatile organic compounds (VOC) with the highest global emission rates. Due to their high reactivity towards OH, monoterpenes can dominate the radical chemistry of the atmosphere in forested areas. In the present study the photochemical degradation mechanism of β-pinene was investigated in the Jülich atmosphere simulation chamber SAPHIR. The focus of this study is on the OH budget in the degradation process. Therefore the SAPHIR chamber was equipped with instrumentation to measure radicals (OH, HO<sub>2</sub>, RO<sub>2</sub>), the total OH reactivity, important OH precursors (O<sub>3</sub>, HONO, HCHO), the parent VOC beta-pinene, its main oxidation products, acetone and nopinone, and photolysis frequencies. All experiments were carried out under low NO<sub><i>x</i></sub> conditions (≤ 2 ppb) and at atmospheric beta-pinene concentrations (≤ 5 ppb) with and without addition of ozone. For the investigation of the OH budget, the OH production and destruction rates were calculated from measured quantities. Within the limits of accuracy of the instruments, the OH budget was balanced in all β-pinene oxidation experiments. However, even though the OH budget was closed, simulation results from the Master Chemical Mechanism 3.2 showed that the OH production and destruction rates were underestimated by the model. The measured OH and HO<sub>2</sub> concentrations were underestimated by up to a factor of two whereas the total OH reactivity was slightly overestimated because of the poor reproduction of the measured nopinone by the model by up to a factor of three. A new, theory-derived first-generation product distribution by Vereecken and Peeters was able to reproduce the measured nopinone time series and the total OH reactivity. Nevertheless the measured OH and HO<sub>2</sub> concentrations remained underestimated by the numerical simulations. These observations together with the fact that the measured OH budget was closed suggest the existence of unaccounted sources of HO<sub>2</sub>.