Measurements of thermophoretic velocities of carnuba wax, polystyrene latex, silver particles and sodium chloride have been performed by injecting the aerosol as a thin sheet into a laminar flow of clean gas, with a temperature gradient established across it. These measurements have enabled us to compare the experimental data with the known theories. The aerosol radius range was 0.11-0.83 mum. The measured reduced thermophoretic velocity was found to depend only on the Knudsen number, in the range 0.09-0.7, and not on the gas/particle conductivity ratio. (C) 2002 Elsevier Science Ltd. All rights reserved.