In this study we examined the co-formation of hydrogen peroxide and other hydroperoxides (collectively presented as H2O2*) as well as submicron particles, including ultra-fine particles (UFP), resulting from the reactions of ozone (O-3) with a complex mixture of volatile organic compounds (VOCs) under simulated indoor conditions. The VOC mixture contained 23 compounds, including two terpenes (d-limonene and alpha-pinene), two unsaturated alkenes (1-decene and 1-octene), and 19 other saturated organic compounds. These compounds are commonly found in indoor air but their concentrations were higher than typical indoor levels. When O-3 was added to a 25-m(3) controlled environmental facility (CEF) containing the 23 VOC mixture, both H2O2* and submicron particles were formed. The 2-h average concentration of H2O2* was 1.89 +/- 0.30ppb, and the average total particle number concentration was 46,000 +/- 12,000 particles cm(-3). A small increase of UFP (0.02-0.1 mu m) occurred 5 min after the O-3 addition (17 min after the VOC addition) and a sharp increase of UFP occurred 13 min after the O-3 addition, suggesting homogeneous nucleation. The delayed onset of this event might reflect the time required to achieve saturated concentrations of the condensable organics. When the 2 terpenes were removed from the O-3/23 VOCs mixture, no H2O2* or particles were formed, indicating that the reactions of O-3 With the two terpenes were the key processes contributing to the formation of H2O2* and submicron particles in the O-3/23 VOCs system. The present study confirmed the findings of a previous study carried out in a real-world office and generated new findings regarding co-formation of UFP. Through a comparative analysis of H2O2* yields under different reaction conditions, this study demonstrates that VOCs co-present with the terpenes and O-3 may play a role in producing H2O2*. (c) 2005 Elsevier Ltd. All rights reserved.