Abstract: A dual UV/IR double wavelength analyser has been developed for the simul-taneous determination of fluorine and hydrogen fluoride in gas. The analyser is composed of the tungsten lamp, the rotating chopper inlayedwith four narrow--band interference filters (2850, 4500, 2.45μm, 2.2μm), thethermistor detector PbS(IR), and photomultiplier(UV). There is a good linear relation between the output of the analyser and thepartial pressure of fluorine or hydrogen fluoride in the range of 0--10, 0--30,0--50, 0--100, and 0--200mmHg. The detection limit of the analyser for fluorineand hydrogen is 0.3mmHg. The analyser is easy to operate, quick and gives a goodreproducibility for measurment. Fluorine and hydrogen fluoride in a mixed gas, HF--F_2--Ar ahd F_2--HF--N_2,have been determined.