Particle inertial deposition in a tube with an abrupt contraction was studied experimentally and numerically at low pressure. Measurements were performed for particle deposition onto an orifice plate (orifice diameter: 1.16 cm) placed in a tube (inner diameter: 3.48 cm). The system pressure ranged from 27 to 37 Pa (0.2-0.28 Torr) and the flow Reynolds number, Re, in the tube was 3.0. Spatially uniform aerosols were produced in the low-pressure environment by the method developed by Sato et al. (Aerosol Sci. Technol., 2001). The observed deposition-efficiency curve as a function of the Stokes number, Stk, is different from that obtained for Re greater than or equal to 100 at atmospheric pressure (J. Aerosol Sci. 21 (1990) 29; J. Aerosol Sci. 26 (1995) 563). The difference was confirmed by numerical studies. The numerical results for Re = 0.1-30 revealed that the deposition curve shifts to a larger Stokes number as the Reynolds number decreases due to flow-pattern changes near the orifice plate. An empirical deposition-efficiency curve was obtained as a function of the Stokes number, the Reynolds number, and the contraction ratio (the ratio of the tube inner diameter to the orifice diameter), R. (C) 2002 Elsevier Science Ltd. All rights reserved.