The product formation from the OH-initiated oxidation of isoprene has been investigated in the presence as well as in the absence of NO x . The experiments were carried out in a 1080l static quartz reactor under atmospheric relevant conditions (room temperature, 900mbar). Irradiation of isoprene/H 2 O 2 /air mixtures was used to study the reaction of isoprene with OH radicals in the absence of NO x . Products were monitored in situ by means of FT-IR-spectroscopy and analysed by GC-FID and GC-MS. Two C 5 -unsaturated dihydroxy compounds, 2- and 3-methyl-3-butene-1,2-diol have been identified by comparison with authentic standards; the yields of these products were (4.7+/-1.4)% and (2.4+/-0.9)%, respectively. The identification of these diols is the first experimental evidence that the self- and cross-reactions of peroxy radicals derived from the isoprene+OH reaction follow the general peroxy radical reaction scheme. An estimate of the potential relevance of these products has been made. The molar yields measured for the well-known main products methacrolein, methyl vinyl ketone and formaldehyde confirm the results from earlier studies. In addition, methanol and methyl hydroperoxide have been observed as minor products in the absence of NO x . New mechanistic information on the OH-initiated oxidation of isoprene has been derived from this study, which helps to improve the understanding of tropospheric isoprene degradation mechanisms.