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Aerosols produced over forests impair visibility and may affect climate by scattering and absorbing solar radiation and by serving as cloud condensation nuclei. Here, we introduce, to our knowledge, a new route to secondary organic aerosol formation from isoprene and its gas-phase oxidation products, methacrolein and methacrylic acid, namely, multiphase acid-catalysed oxidation with hydrogen peroxide, a perfect analogue to atmospheric sulphate formation. We demonstrate that the formation of major secondary organic aerosol components that are present in natural forest aerosols collected at K-puszta, Hungary, during the summer of 2003, namely, 2-methyltetrols and 2,3-dihydroxymethacrylic acid, can be explained by this mechanism. [Copyright &y& Elsevier]