Simultaneous measurements of hydroperoxides in both the gas- and the aerosol-phase have been made for the first time. In addition, hydroperoxide levels in the 'coarse' ( > PM2) and 'fine' (PM2) aerosol modes have been characterized. Hydrogen peroxide (H2O2) was found to be the major hydroperoxide present, with trace amounts of methyl hydroperoxide (CH3OOH, MHP) occasionally being observed. Between May and August 2001, ambient gas- and aerosol-phase hydroperoxide levels were in the range 0.5-3.5 ppbv, and 0-13 ng m(-3), respectively. On average, about 40% of aerosol-phase H2O2 was associated with fine particles. The observed aerosol mass loadings correspond to aqueous concentrations of 10(-4)-10(-3) M, above the levels necessary to induce lung epithelial cell damage in laboratory studies. The measured H2O2 mass loadings were found to be several times larger than could be explained by the solubility of gaseous H2O2 in liquid water, Potential reasons for this discrepancy are briefly discussed. (C) 2003 Elsevier Science Ltd. All rights reserved.