The tandem differential mobility analyzer (TDMA) technique was used to measure evaporation rates of monodisperse organic aerosol droplets in the 0.02- to 0.2-μm-diameter range. The species investigated include dioctyl phthalate (DOP), diocytl sebacate (DOS), and oleic acid, all of which are frequently used to produce monodisperse aerosol standards for instrument calibration. Transition regime mass transfer expressions were used to determine vapor pressures from measured evaporation rates. The vapor pressure of DOP at 25.7°C was found to be in good agreement with reported values (2.36 × 10−4 dynes/cm2). Vapor pressures at 25° C for oleic acid and DOS were 2.2 × 10−4 dynes/cm2 and 2.6 × 10 −5 dynes/cm2, respectively. The data provide direct evidence for the validity of the Kelvin equation for these species. Also, these results suggest that because of its lower vapor pressure, DOS is more stable than the other species, and is therefore more suitable for use as a material for monodisperse aerosol standards.