A new technique is presented to measure the concentration of fluorine in a reaction gas consisting of fluorine, nitrogen, and oxygen in a reaction chamber. The method is described as follows: The gas is directly taken from the chamber using a peristaltic pump and dissolved in a TISAB (total ionic strength adjusted buffer) solution. The concentration of fluorine ion is measured using the selected fluorine ion electrode, the output voltage of which is monitored. The concentration of the fluorine ion and the output voltage follow the Nernst relation so that the measurement of the output voltage of the system enables us to know the concentration of fluorine gas. This technique was applied to monitor the concentration of fluorine gas in the chamber in which a sheet of polyethylene nonwoven fabric was continuously oxy-fluorinated. The monitored concentration of the fluorine gas as a function of time coincided with the variation of the incorporated fluorine in the sheet measured by X-ray fluorescence (XRF) spectroscopy after the treatment. This technique can be best used for process control of the continuous oxy-fluorination of polymeric materials.