A hydrophobic polymer material is presented which is optically sensitive to carbon dioxide. It is based on ion pairing between an indicator anion and a quatemary ammonium cation. The ion pair is dissolved in various kinds of silicone rubbers, and the resulting materials are shown to be useful for optically sensing carbon dioxide over the O-100 hPa (O-76 Torr, O-100 mbar) partial pressure range. The detection limit is approximately 0.3 hPa. The material can be immobilized on glass or plastic supports, on optical fibers, or in capillaries. A new method for making ion pairs is described which yields sensor materials containing no inorganic ions. This makes the ion pairs soluble even in highly apolar silicones. The silicone rubber based sensors are capable of measuring carbon dioxide both in gases (to which they respond with a To" of 1 s) and in aqueous sample solutions (with response times ranging from 2 to 8 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 5-9 range. Shelf lifetimes of more than 5 months are found when membranes are stored in sealed plastic bags, but sensors need to be recalibrated. The operational lifetime is in the order of 2 months when used for sensing carbon dioxide in potable water, provided the sensor is not in contact with acidic vapors such as sulfur dioxide or gaseous hydrogen chloride which can be present in many laboratories.