Atmospheric hydrogen peroxide (H<sub>2</sub>O<sub>2</sub>) and organic hydroperoxides were measured from 18 to 30 July in 2006 during the PRIDE-PRD'06 campaign at Backgarden, a rural site located 48 km north of Guangzhou, a mega-city in southern China. A ground-based instrument was used as a scrubbing coil collector to sample ambient air, followed by on-site analysis by high-performance liquid chromatography (HPLC) coupled with post-column derivatization and fluorescence detection. The H<sub>2</sub>O<sub>2</sub> mixing ratio over the 13 days ranged from below the detection limit to a maximum of 4.6 ppbv, with a mean (and standard deviation) of (1.26±1.24) ppbv during the daytime (08:00–20:00 LT). Methyl hydroperoxide (MHP), with a maximum of 0.8 ppbv and a mean (and standard deviation) of (0.28±0.10) ppbv during the daytime, was the dominant organic hydroperoxide. Other organic peroxides, including bis-hydroxymethyl hydroperoxide (BHMP), peroxyacetic acid (PAA), hydroxymethyl hydroperoxide (HMHP), 1-hydroxy-ethyl hydroperoxide (1-HEHP) and ethyl hydroperoxide (EHP), were detected occasionally. The concentration of H<sub>2</sub>O<sub>2</sub> exhibited a pronounced diurnal variation on sunny days, with a peak mixing ratio in the afternoon (12:00–18:00 LT), but lacked an explicit diurnal cycle on cloudy days. Sometimes a second peak mixing ratio of H<sub>2</sub>O<sub>2</sub> was observed during the evening, suggesting that H<sub>2</sub>O<sub>2</sub> was produced by the ozonolysis of alkenes. The diurnal variation profile of MHP was, in general, consistent with that of H<sub>2</sub>O<sub>2</sub>. The estimation indicated that in the morning the H<sub>2</sub>O<sub>2</sub> detected was formed mostly through local photochemical activity, with the rest probably attributable to vertical transport. It is notable that relatively high levels of H<sub>2</sub>O<sub>2</sub> and MHP were found in polluted air. The unexpectedly high level of HO<sub>2</sub> radicals detected in this region can account for the production of hydroperoxides, while the moderate level of NO<sub>x</sub> suppressed the formation of hydroperoxides. High concentrations of hydroperoxides were detected in samples of rainwater collected in a heavy shower on 25 July when a typhoon passed through, indicating that a considerable mixing ratio of hydroperoxides, particularly MHP, resided above the boundary layer, which might be transported on a regional scale and further influence the redistribution of HO<sub>x</sub> and RO<sub>x</sub> radicals. It was found that hydroperoxides, in particular H<sub>2</sub>O<sub>2</sub>, play an important role in the formation of secondary sulfate in the aerosol phase, where the heterogeneous reaction might contribute substantially. A negative correlation between hydroperoxides and water-soluble organic compounds (WSOC), a considerable fraction of the secondary organic aerosol (SOA), was observed, possibly providing field evidence for the importance of hydroperoxides in the formation of SOA found in previous laboratory studies. We suggest that hydroperoxides act as an important link between sulfate and organic aerosols, which needs further study and should be considered in current atmospheric models.