The quasi-real-time measurement of ultrafine aerosol size distributions by a combination of a diffusion screen separator (DS), which is similar to a single stage of a screen-type diffusion battery, and a condensation particle counter (CPC) was developed, characterised, and evaluated. The instrument reports information on the particle size distribution for particle diameters between approximately 20 and 100 nm. The instrument system was characterised for various operation conditions in order to provide reliable instrument transfer functions for data inversion purposes. The penetration behaviour of specified diffusion screens was examined for the expected range of operation pressure conditions (200 hPa ≤ p ≤ 955 hPa), number of diffusion screens (3 ≤ nS ≤ 17), and volumetric flow rates (0.99 ≤ q ≤ 1.89 Lpm). The comparison of results between the calibration experiments and the "classical" filtration theory of particle deposition on fibres or wires shows excellent agreement for the entire range of investigated operation conditions. The calibration results were evaluated in a validation experiment using monodisperse test aerosol particles. In a final study, the entire measurement system consisting of three parallel-operated CPC–DS combinations was successfully applied to polydisperse laboratory-generated test aerosols. A comparison of size distributions obtained from a differential mobility analyser (DMA) and the presented instrument shows very close agreement for cases with the modal diameter of the investigated aerosol size distribution being in the sub-100 nm range.