Biomass fast pyrolysis vapors contain a significant quantity of persistent aerosols, which can impact downstream processing by e.g. fouling of surfaces and deposition on downstream catalysts. In this study, aerosol concentrations and size distributions were measured by an impactor in two pyrolysis systems, a bench-scale fluidized-bed pyrolyzer and a pilot-scale cyclone pyrolyzer. In both units, the mass-based mode aerosol diameter was approximately 1 um before aerosol collection devices in cooled vapors of 300-370 K but the number-based median was <
0.1 um. Aerosols <
1 um were formed and aerosols >
1 um deposited during cooling of pyrolysis vapors from 620 to 370 K in the fluidized-bed pyrolysis system. Here, the oil fraction collected from the aerosols constituted approximately 40 wt% of the total oils collected in both systems. Compared to the total collected oil, the oil fraction from the aerosols was enriched in lignin-derived components and anhydrosugars and had lower concentrations of low molecular weight cellulose derived oxygenates, such as hydroxyketones.