Mass spectrometry has proven valuable in understanding and controlling chemical processes used in semiconductor fabrication. Given the complexity of spatial distributions of fluid flow, thermal, and chemical parameters in such processes, multipoint chemical sampling would be beneficial. The authors have designed and implemented a multiplexed mass spectrometric gas sampling system for real-time, in situ measurement of gas species concentrations at multiple locations in a spatially programmable chemical vapor deposition (SP-CVD) reactor prototype, where such chemical sensing is essential to achieve the benefits of a new paradigm for reactor design. The spatially programmable reactor, in which across-wafer distributions of reactant are programmable, enables (1) uniformity at any desired process design point, or (2) intentional nonuniformity to accelerate process optimization through combinatorial methods. The application of multiplexed mass spectrometric sensing is well suited to our SP-CVD design, which is unique in effectively segmenting the showerhead gas flows by using exhaust gas pumping through the showerhead for each segment. In turn, mass spectrometric sampling signals for each segment are multiplexed to obtain real-time signatures of reactor spatial behavior. Here the authors report results using inert gases to study the spatial distributions of species, validate SP-CVD reactor models, and lead to an understanding of fundamental phenomena associated with the reactor design. This forms the basis for using real-time mass spectrometry to drive process sensing, metrology, and control in such reactor systems. (c) 2007 American Vacuum Society.