The influence of the WF6 concentration on the growth rate in tungsten LPCVD from WF6 and H2 has been studied bothexperimentally in a coldwall single-wafer reactor and with the use of a mathematical simulation model, predicting the gasflow, heat transfer, species transport, and chemical reactions in the reactor. Model predictions were in very good agreementwith experimental growth rates and uniformities. The growth rate was found to be independent of the WF6 inletpressure above a certain value Pcrit, whereas for WF6 inlet pressures below Pcrit the growth rate decreases linearly with theWF6 inlet pressure. It is shown that this transition is due to mass-transfer limitations rather than a change in the reactionmechanism. The value of Pcrit depends on the reactor geometry and process conditions and may be obtained experimentallyor from model simulations as presented in this study. It is shown that large concentration gradients may be present inCVD reactors, even at low reactant conversion rates, and that criteria for "gradientless" reactor operation based on conversionrates are incorrect. We propose a better criterion, based on the value of Pcrit. It is also shown that thermal diffusionphenomena in coldwall reactors are very important. As a result, WF6 concentrations at the wafer surface will always be significantlylower than the inlet concentration.