The super ionic conductor LaF3 is the essential part of an n-Si-SiO2/Si3N4/LaF3/Pt sensor structure for the detection of fluorine in air. The semiconductor sensor, which is based on the field effect in the silicon substrate, was fabricated using thin film technology. The sensitivity depended on the Pt layer thickness resulting in a value of 116 mV/dec for 30 nm Pt films. The results of impedance spectroscopy showed the electrochemical ion exchange at the three-phase boundary LaF3/Pt/gas to be the rate-determining step of the sensing process. A fluoride ion is introduced into the ionic conductor LaF3. The interaction of Pt with fluorine gas at concentrations of 1000 ppm was investigated using Auger electron spectroscopy and a quartz crystal microbalance. It was shown that a nanometer-scale thin film of PtF6 was formed. The formation of this film was the reason for the extremely high sensitivity.