We have synthesized diamond films in a flow tube system in which methane is added downstream from a microwave discharge in 90% argon/10% hydrogen at 3 Torr. The chemistry in this system is simplified because the methane is not exposed to temperatures higher than the 850 °C wall temperature of the flow tube. Therefore, only the reaction of hydrogen atoms with methane initiates the chemistry of this system. Furthermore, the system is at uniform temperature and flow speed. Under these conditions, diamond is deposited on silicon substrates for a time of about 1 ms after injection of the methane.