Abstract Molecular beam techniques are applied to measure fundamental properties of aerosol particles. An aerosol-laden gas is sampled with a microprobe or a capillary tube and drawn into a vacuum chamber, where gas molecules are pumped out and an aerosol beam is formed. A surface ionization technique is used to measure the particle number intensity in aerosol beams of alkali metal compounds (primarily potassium iodide). The particle velocity is measured by a time-offlight technique, and the particle mass is measured by electrically charging the particles and subsequently deflecting them in an electrostatic field. Velocity relaxation in a retarding gas is utilized to measure the particle size, and the electric polarizability of neutral aerosol particles is measured by deflecting them in a nonuniform electrostatic field. Particle velocities from 70 to 670 m/sec masses from 1 × 10 −20 to 1 × 10 −18 kg, and radii from 0.01 to 0.1 μm were measured. Anomalously low mass densities and high electric polarizabilities were observed for these particles. This work demonstrates that aerosol beam techniques are potentially valuable tools for fundamental studies of aerosol particles.