A flexible sensor tube for determining a concentration curve of a medium along a path includes a flexible tube being formed of a material having good permeability to the medium and having an inner surface being completely provided with a thin coating of a material having poor permeability to the medium. A flexible tube of a material having good permeability to the medium may also be coated on the inner surface with a material being impermeable or having poor permeability to the medium, with a narrow strip extending in the longitudinal direction of the flexible tube being left free. In addition, a band being formed of a material being impermeable or having poor permeability to the medium, can be bent into the shape of a trough, while a slot-like aperture remains free. The slot-like aperture is closed by a strip being formed of a material having good permeability to the medium and extending in the longitudinal direction of the band.