Fiber-optic sensors were developed for monitoring dissolved carbon dioxide in water samples in the 0 to 900 ppm concentration range. A pH-sensitive fluorescent dye (HPTS) was reacted with a cationic quaternary ammonium salt to form an ion pair which was electrostatically bound to the surface of particles of aminocellulose which then were dispersed into a gas-permeable silicone polymer. The green fluorescence of the base form was monitored using a fiber optic fluorometer. The use of the aminocellulose enhanced the stability and lifetime of the sensor and also increased the fluorescence of the sensor membrane because the particles act as scattering centers. The characteristics of the sensors are described with respect to dynamic range, reproducibility, long-term stability and temperature dependence.