Measurements of particle nucleation following the gas phase oxidation of α-pinene and β-pinene are reported. Particle nucleation following ozonolysis and O3 plus OH initiated monoterpene oxidation was measured in a 70-L Teflon bag reactor over the temperature range 278 to 320 K. Particle concentration temporal profiles were measured for a range of initial monoterpene and ozone concentrations using ultrafine condensation particle counters. Profiles were interpreted using a coupled gas phase chemistry and kinetic multicomponent nucleation model to determine the molar yield of the nucleating species in the ozonolysis experiments to be 1 × 10−5 and 0.009, for α-pinene and β-pinene, respectively. OH initiated oxidation was found to increase the nucleator yield for α-pinene, approximately a factor of three, but not for β-pinene. The molar yield of condensable reaction products (vapor pressure < 20 ppt at 296 K) was determined to be 0.06 for both α-pinene and β-pinene and was independent of the oxidation source. Particle growth, which is determined by the condensable reaction products, was nearly temperature independent. Atmospheric box model calculations of nucleation and particle growth for α-pinene and β-pinene oxidation under typical tropospheric conditions are presented and showed that (1) nucleation can be significant under favorable conditions, (2) nucleation is dominated by OH initiated oxidation, and (3) the partitioning of the condensable monoterpene oxidation products made a significant contribution to the growth of atmospheric aerosol and was capable of explaining the observed particle growth rates commonly observed in remote forests.