We investigate the uptake of the most prominent biogenic volatile organic compounds (VOCs)-isoprene, α-pinene, and their selected oxidation products-by hydrated acid clusters in a molecular beam experiment and by DFT calculations. Our experiments provide a unique and direct way of determination of the surface accommodation coefficient (αS) on the proxies of ultrafine aerosol particles. Since we are able to determine unambiguously the fraction of the clusters to which the molecules stick upon collisions, our αS is a purely kinetic parameter disentangling the molecule pickup from its evaporation. Oxidation increases the αS of VOCs by more than an order of magnitude, because oxidized compounds form hydrogen bonds with the clusters, whereas the interactions of the parent VOCs are weaker and nonspecific. This work provides molecular-level insight into the condensation of single molecules into atmospheric particles, which has important implications for aerosol nucleation and growth.