Steady and transient mass transport phenomena within an inverted, stagnation-flow, cold-wall CVD reactor were investigated by observing the concentration of a tracer species (CH4) with in situ Raman spectroscopy. The transient studies revealed that the use of matched reactor inlet velocities is crucial to minimize recirculating flow patterns and that the magnitude of the gas velocity is also important in gas switching. In the steady-state studies, it was observed that the existence of a sufficiently large density gradient in the reactor initiates natural convection and under some conditions introduces a flow instability and thus three-dimensional (3-D) flows. The onset of instability was characterized by solutal density difference, gas velocity, and distance traveled by the gas. A steady-state, 2-D axisymmetric reactor model validated with experimental data was used to analyze the measured tracer concentration profiles. (c) 2005 Published by Elsevier B.V.