A single layer hydrophobic polymer membrane is presented which is optically sensitive to carbon dioxide. It is immobilized on the inner surface of a glass capillary. The sensor material is based on ion pairing between an indicator anion (D-) and a quaternary ammonium cation (Q+). The ion pair (Q+D-), together with a quaternary hydroxide (Q+OH-), is dissolved in silicone rubber, and the resulting material is shown to be useful for optically sensing carbon dioxide over the 0 to 100 hPa (0 to 76 Torr) partial pressure range. The detection limit is approximately 0.3 hPa. The silicone rubber based sensors are capable of measuring carbon dioxide both in gases (to which they respond with a t90 of 1 sec) and in aqueous sample solutions (with response times ranging from 1 to 2 min); there is no need for an additional (proton-impermeable) coating. Due to the use of silicone rubber as the polymer support, there is no cross- sensitivity to pH within the pH 5 to 9 range. We found the inner surface of glass or plastic capillaries to be most suitable as a support for the sensor chemistry. Such capillaries can act as a mechanical support, they represent an optical waveguide structure and therefore enable various methods of optical interrogation, and they can serve as a sample cavity of well defined volume, which is suitable for direct sampling, or as a flow cell. Sensitivity, which determines the degree of response of the sensor to a given analyte concentration, is a vital parameter for the performance of a sensor. In this work, it is shown that the sensitivity of carbon dioxide sensors based on ion pairing largely depends on the molar (ion pair)/(free quaternary hydroxide) ratio. We have found that the sensitivity of such optrodes was well as their degree of linearity can be adjusted by varying the (Q+D-)/$Q++)OH-) ratio.