A particle beam mass spectrometer has been developed for measuring the size distribution of submicron (in this study ∼ 0.02-0.3 μm) particles in low-pressure environments. Particles are sampled into a differentially pumped system and focused into a narrow, low-divergence beam using aerodynamic lenses, charged to saturation in an electron beam, and then deflected electrostatically into a Faraday cup for measurement of the particle current. Measurements of particle current as a function of deflection voltage are combined with measured relationships between particle velocity and diameter, charge and diameter, and mass and diameter, to determine the particle size distribution. Results of measurements on KCl particles are in good agreement with size distributions obtained by differential mobility analysis. The maximum diameter resolution ( Dp ΔDp) is about 10, a complete size distribution can be measured in about 2-3 min, and the detection limit is about 10 particles cm-3 when sampling from a pressure of 270 Pa.