An on-line, semicontinuous method for the determination of size and effective density (g/cm3) of particles in the 3–50 nm size range was evaluated using laboratory test aerosols. The method, called double size spectrometry, sizes particles twice, first by electrical mobility, and then by aerodynamic behavior. The first measure gives the geometric diameter for spherical particles. The second measure depends on single particle density. The double size spectrometer uses a differential mobility analyzer and hypersonic impactor in series. The mobility analyzer outputs singly-charged particles of a uniform diameter that are resized by the impactor. The impactor flow is supersonic; the critical aerodynamic capture diameter is varied by adjusting the orifice-to-plate separation; and particle capture is measured on-line by an electrometer. Theoretically, the response is described by a particle Stokes number. Experiments with 10–40 nm particles of dioctylsebacate and a perfluorocarbon vacuum oil gave critical Stokes number for capture near 1. The measured density ratio was 1.9, compared with the bulk material density ratio of 2.0. Experiments with 3–14 nm spark generated silver and copper aerosols differed, with apparent aerosol densities lower than the bulk values by factors of 2.5–3.6.