The direct polymerization of isoprene and α-pinene on acidic sulfate aerosols has been studied in a reaction chamber utilizing aerosol mass spectrometry. Results indicated that both species can be directly taken up into acidic aerosols to a significant extent, forming polymers that contain at least 4 isoprene or 2 α-pinene repeating units. Aerosol mass spectra indicate that double bonds in the polymers hydrate under acid catalysis, leading to partial oxygenation of the polymers. This reactive uptake depends highly upon relative humidity and particle acidity. This process is rapid and reaches equilibrium in less than 50 minutes, with effective partition coefficients (K p,eff ) between 1.2--14.1 W 10-6 m3 5g-1, from which it is estimated <0.5--5 ng m-3 of polymers may be present from both species in acidic aerosols. The formation of biogenic polymers is an important mechanism for incorporating hydrophobic, unsaturated species into polar aerosols and enhanced SOA formation.