<p>Oligomeric hydroperoxides, including stabilized Criegee intermediates generated during isoprene ozonolysis, play an important role in new particle formation (NPF). In this study, we experimentally determined the relative abundance (<i>Φ</i>^NPF) of new particles formed during isoprene ozonolysis, competing against the growth of preexisting particles. The number concentration of newly formed particles (<i>N</i>^NPF) during isoprene ozonolysis was derived by comparing the size distribution of secondary organic aerosols (SOAs) in the presence of seed particles with that under humid conditions (relative humidity (RH) &gt; 20%) at the same reaction time. The number concentration of particles that took up semi-volatile organic compounds (<i>N</i>^uptake) was estimated from the difference in the size distribution between particle wall loss (PWL)-considered seed particles and SOAs with seed particles under humid conditions. The <i>Φ</i>^NPF was then calculated using the formula: <i>N</i>^NPF/(<i>N</i>^NPF + <i>N</i>^uptake) under different conditions. The methodology to determine the <i>N</i>^NPF was generally successful, whereas the determination of <i>N</i>^uptake was complicated due to the instability of PWL in the small Teflon bag experiments. The <i>Φ</i>^NPF can be represented as a product of the <i>r</i>^NPF(RH), the relative abundance of new particles formed during isoprene ozonolysis as a function of RH, and the <i>ϕ</i>^NPF(dry), the <i>Φ</i>^NPF value obtained under dry conditions. The obtained <i>r</i>^NPF(RH) values suggested that NPF can occur only under very limited RH conditions (RH &lt; 10%) of isoprene ozonolysis in the atmosphere, but the products from the reaction of isoprene with O₃, probably Criegee intermediate oligomerization products, were found mainly to contribute to NPF.</p>