The article presents a new idea of the application of polymer track membranes (PTM) for immobilization of a reagent in fiber optic chemical sensors. PTM was made of a poly(ethylene terephthalate) foil (10 mu m in thickness, pores of 0.2 mu m in diameter). The usefulness of membranes additionally covered by poly(vinyl chloride) was tested in a fiber optic redox titrator. The titrator utilized N,N’-diphenylloenzidine as a reagent which changes its absorbance in dependence on the redox potential. The measuring system is based on a light-emitting diode and a silicon photodiode connected to a bifurcated fiber optic bundle. The gain is in price and availability of the membrane.