An optical probe for the invasive measurement of the partial carbon dioxide pressure comprises an optical fiber (18)' and a pC0 2 sensor (15'). The pC0 2 sensor (15') comprises a diffusion zone with a dye-containing gel (23') which is optically sensitive to H ions and a semi-permeable membrane (30') which is permeable to C0 2 molecules, but basically not to hydrogen ions. Components of the pC0 2 sensors such as the part of the optical fiber (18') extending into the sensor or a reflector (24') are covered with a substance - preferably, with a coating (35, 36) - that is basically non-permeable to hydrogen ions. This prevents the hydrogen ions from diffusing into the sensor components, which otherwise would cause an imbalance of hydrogen ions and therefore a drift in the pC0 2 reading.