A plate-to-plate thermal precipitation system has been used to investigate the deposition of agglomerate submicrometer soot aerosol particles under flow and temperature conditions relevant for modem heavy-duty diesel engine exhaust gas systems. Over a flow channel length of 45 cm, thermophoretic particle deposition efficiencies of 4-95% have been observed as a function of plate distance (0.45-1.45 mm), temperature gradient (10(4)-10(5) K m(-1)), and flow velocity (2-11 m s(-1)). Throughout the investigated particle size range of 34-300 nm (electrical mobility diameter), the observed deposition efficiencies are nearly independent of particle size and Knudsen number. The experimental results are in good agreement with theoretical calculations assuming a constant thermophoretic coefficient K-th = 0.55 which had been derived by Waldmann and Schmitt (in C. N. Davies (Ed.), Aerosol Science, London, 1966) for the free molecular regime (Kn much greater than 1). Our results demonstrate that K-th approximate to 0.55 is also applicable for agglomerate soot particles in the transition regime (Kn approximate to 1), which is supported by a recent theoretical study by Rosner and Khalil (J. Aerosol Sci. 31 (2000) 273) on the effect of reduced thermal conductivity on the thermophoretic coefficient of agglomerate particles. To our knowledge, the measurement data presented in this paper provide the first direct experimental validation for the theory of Rosner and Khalil under conditions relevant for diesel engine exhaust gas treatment. Moreover, they confirm the applicability of a simple formula of approximation for the thermophoretic deposition efficiency derived by Tsai and Lu (Aerosol Sci. Technol. 22 (1995) 172). (C) 2003 Elsevier Ltd. All rights reserved.