In the atmosphere, cloud and fog droplets usually contain insoluble material. The role of these insoluble particles is still unknown today, and is of interest to study. To determine the size distribution and number concentration of these particles in water, different techniques are available. The instrumentation, however, to measure nanometer-sized particles down to 50 nm diameter is not known. A new instrument, the Liquid Tandem Differential Mobility Analyser (LTDMA), was developed to measure size distributions of insoluble particles in water in the size range 50–300 nm in diameter. The new method is based on nebulising, e.g. cloud water and forming a residue aerosol consisting of both, insoluble particles with a soluble shell, and pure soluble particles. The insoluble, hydrophobic particles can be separated from soluble, hygroscopic residue particles with a Tandem Differential Mobility Analyser. The system is calibrated with monodisperse latex particles to determine the size-dependent transmission factor of insoluble particles of the Liquid Tandem Differential Mobility Analyser. A size distribution of insoluble particles in cloud water is presented as an application of this new measuring method. The cloud water sample was taken during the field campaign of the EUROTRAC sub project Ground-based Cloud Experiments (GCE) on the Kleiner Feldberg mountain, Germany, in November 1990.