Convective, diffusive, and thermophoretic particle transport in a parallel-plate semiconductor reactor is investigated. Measurements that illustrate particle transport in the reactor are presented and a Eulerian continuum particle transport formulation is used to quantitatively explain the measurements. Experimental and numerical results show that particles formed in the parallel-plate region are confined in a thin sheath (similar to 2 cm) between the "hot" water and "cold" showerhead inlet. This sheath is located at the point where downward convective transport balances upward transport by thermophoresis. The particle sheath location is independent of particle size but is dependent on gas flow rates and temperature of the wafer and showerhead inlet. In addition, experimental and numerical results show that as particles exit the parallel-plate region, the radial thermophoretic particle transport can produce "ring-like'' contaminant deposits on the outer wall of the reactor under certain flow conditions. We propose a simple reactor design modification and an analytic design criterion to avoid particle deposition on the chamber walls.