The reactions of ozone with a series of monoterpenes (α-pinene, sabinene, limonene and myrcene) were investigated in a novel flow reactor dedicated to the investigation of secondary organic aerosol (SOA) formation. Rate constants for the gas phase reactions and nucleation thresholds were determined at T∼296 K, P∼764 Torr under dry conditions (dew point ≤−33 °C) and in absence of OH radicals scavenger and seed particles. Comparison with the literature as well as data from a simulation chamber showed good agreement. The experiments also show that the novel flow reactor improves the accuracy in evaluating the nucleation thresholds during the ozonolysis of monoterpenes and show that aerosol flow reactor is a useful tool to study the SOA nucleation step. Given as an upper limit, the nucleation thresholds obtained are (in molecule cm−3/ppb): α-pinene, 3.9×1010/1.56; sabinene, 6.2×109/0.26; limonene, 1.1×1010/0.43 and myrcene 2.1×1010/0.83.