The effects of composition, size, and phase state on ultrafine seed particle growth by α-pinene ozonolysis were determined from diameter growth measurements after a fixed reaction time in a flow tube reactor. Modeling time-dependent particle growth under a given set of conditions allowed the reaction growth factor (GF) to be determined, which is defined as the fraction of α-pinene molecules that react to give a product that grows the particles. Growth factors were compared for initial seed particle diameters of 40, 60, and 80 nm that were composed of freshly formed α-pinene secondary organic aerosol (SOA), effloresced ammonium sulfate, and deliquesced ammonium sulfate. Overall, SOA seed particles gave the lowest growth factors. Effloresced ammonium sulfate particles gave somewhat higher growth factors and showed a slight dependence on relative humidity. Deliquesced ammonium sulfate particles gave the highest growth factors. Seed particle-size dependencies suggested that both surface-and volume-limited reactions may contribute to growth. Overall, the growth factors were found to vary by more than 4x across the reaction conditions studied. The results highlight the crucial role that seed particle characteristics play in determining particle growth rates in a size range relevant to formation of cloud condensation nuclei.