a method of measuring the concentration of carbon dioxide in a medium by determining the water content in a ph - independent sensor system comprising an optical fiber having attached thereto a fluorescence indicator embedded in a carrier matrix containing a miscible mixture of water and non - aqueous solvent in predetermined proportions and being separated from said medium by a gas - permeable , water - impermeable diffusion membrane , said indicator , when excited by excitation radiation of a predetermined wavelength , emitting fluorescent emission whose intensity is dependent upon the ratio of water to non - aqueous solvent present in the system such that the ratio of intensities of emitted radiation is therefore proportional to the amount of water present and diffusion of carbon dioxide through said gas - permeable membrane and subsequent reaction with water to deplete the water content of the system induces a change in the intensities of said emissions , which method comprises transmitting excitation radiation through said optical fiber and measuring the ratio of the intensities of the emitted radiation , thereby obtaining a determination of the water content and calculating therefrom the carbon dioxide concentration in the surrounding medium .