The dependence of the density of atomic fluorine as a function of pressure and flow rate in CF4 glow discharges has been measured using emission spectroscopy. The emission intensity of the 7037 Å line of atomic fluorine and of the 7504 Å line of argon, which is added to the CF4 discharge in small quantities (1%–2%), is used to deduce the F atom density. It is found that the pressure dependence of the F atom emission intensity alone is a poor indicator of the relative F atom concentration because the excitation efficiency of the CF4 discharge decreases rapidly with increasing pressure. The F atom density decreases with increasing flow rate as is to be expected if the lifetime of the F atom is determined largely by the vacuum pump. However, at low flow rates the F atom density is determined primarily by contaminant gases, in this case CO and CO2, through several mechanisms which are discussed.