A wide range of chemical and physical parameters has been observed over the course of a severe Europe-wide air pollution episode in August 2003. Detailed surface observations made at the rural perimeter edge of London, UK, indicated significantly elevated levels of primary volatile organic compounds (VOCs), ozone (>110 ppbV), other photochemical by-products such as PAN, HCHO, and oxygenates but not NOx. Reactive tracers in combination with Doppler wind radar and back trajectories have been used to establish that initial morning rises in O3 during the episode were caused by entrainment of air from aloft, polluted on regional scales from mainland Europe. Total VOC reactivity to OH approximately doubled during this episode, with similar distribution between functional groups, but showing a temperature dependent contribution from biogenic isoprene (maximum 1.2 ppbv). Under episodic conditions total peroxy radicals in excess of 60 pptv were observed in late afternoon with strong correlation to a later and higher peak in ozone. Episodic alkyl peroxy radicals were calculated to be substantially influenced by peroxy acetyl, via the thermal dissociation of PAN at high temperatures.