The reaction of WF6 with Si has been studied by time-resolved mass spectrometry, monitoring the reaction product SiF4. In the time development of the deposition, two stages are distinguished. In the first stage, a reaction layer consisting of W and SiFn species is formed. The total amount of Si that will be consumed is determined in this stage. In the second stage, the determined amount of Si desorbs as SiF4, after addition of the required F atoms. The rate limiting step is an energy-activated process with an activation energy of 1.6 ± 0.2 eV, probably the addition of F to SiF3.