Particle size distributions are of profound interest in the study of ambient aerosols. Electrostatic classification using the Scanning Mobility Particle Sizer (SMPS) and more recently the Fast-Mobility Particle Sizer (FMPS) is the most commonly employed approach to establish particle size distributions for submicron particles in field and laboratory applications. The FMPS enables fast size distribution measurements on a timescale of seconds but has been speculated to underestimate particle size. Aerosol mass spectrometry has emerged as another well-accepted method for size-resolved compositional aerosol analysis with particle sizing being accomplished by flight time separation over a specified flight path under vacuum conditions. In this work, we characterized the particle sizing performance of an FMPS against simultaneous measurements with an Aerodyne Aerosol Mass Spectrometer (AMS) and an SMPS by sampling ambient particles, as well as polydisperse and monodisperse particles from aqueous inorganic salt solutions in the size range from 50 nm to 450 nm. The particle size measurements by AMS and SMPS produced similar results, while the FMPS significantly underestimated particle size by 40–50%. The discrepancy was observed in all studied ambient and laboratory-generated aerosols and appeared to be largely independent of the sampled species. The observations suggest that it is crucial to evaluate the sizing performance of the FMPS against other instruments to ensure an adequate accuracy of the particle size measurements. In this study, a simple postcorrection method for the FMPS measurements was applied, which was able to successfully reduce the initial underestimation.Copyright 2013 American Association for Aerosol Research