Gas-phase reactions leading to radical chain reactions of tungsten hexafluoride (WF6) and silane (SiH4) in tungsten silicide (WSix) chemical vapor deposition (CVD) were studied using a hot-wall tubular reactor. To prevent film growth being limited by gas-phase diffusion or by surface reactions, a reactor diameter from 2.4 to 4 mm and pressures from 0.5 to 10 Torr were used. To extend the reaction zone to permit detailed measurement of the axial film growth-rate profile, relatively high axial flow velocities from 6 to 19 m s(-1) were used. A two-dimensional numerical simulation was used to improve the accuracy of the analysis. The measured overall reaction rate, r, was independent of pressure and reactor diameter, indicating that it could be expressed by a simple first-order reaction of WF6, r = k(gr) C-WF6, where C-WF6 is the WF6 concentration. An Arrhenius plot of k(gr) gave an activation energy of 28 kJ mol(-1). This relatively small activation energy confirms that the gas-phase reactions are controlled by radical chain reactions. The experimentally observed behavior that the reaction rate was independent of SiH4 concentration may suggest that SiH4 does not participate in the elementary reaction that activates WF6. One possibility is that thermal decomposition of WF6 limits the initiation reaction.