The theory, construction and performance of a catheter tip optical PCO2 probe is described. The sensor, called the Opticap, is made with plastic fiber optics. One fiber carries light to the sensitive tip which is a silicone rubber tube 0.6 mm dia. X 1.0 mm long filled with a phenol red-KHCO3 solution. Ambient PCO2 controls the pH of the solution which influences the optical transmittance of the phenol red. A second fiber carries the transmitted signal to a receiver; the resulting electrical signal is linearly related to the PCO2 over the range of 2.7 to 10.7 kPa. The probe was tested as a tissue PCO2 sensor on the cerebral cortex of the cat and as an arterial PCO2 sensor. Drift over one day's use was 0.6 KPa or less and individual probes have been used as long as 12 weeks.