We report the first time-dependent measurements of high-molecular-weight (up to 700 amu) gas-phase oxidation products from α-pinene ozonolysis in an aerosol chamber under dry and low-NOx conditions. Measurements of products having mole fractions ranging from 10−14 to 10−11 were carried out with a chemical ionization mass spectrometer (the Cluster CIMS). Most products that were correlated with number concentrations of the smallest particles measured (10-20 nm) had molecular weights in the 430-560 amu range. Those products are proposed to be likely responsible for the initial nuclei formation and the early growth of the freshly nucleated particles based on their high molecular weights and chemical identities, both of which suggest low-volatility compounds. Another group of oxidation products in the lower mass range of 140-380 amu was well correlated with particles larger than 20 nm. We postulate that those products contributed to the later growth of particles (i.e., larger than 20 nm in diameter). Although particle nucleation in this study was primarily due to condensation of oxidation products from α-pinene ozonolysis, the involvement of residual sulfuric acid vapor in particle nucleation cannot be totally excluded.