A probe for the direct measurement of the partial pressure of carbon dioxide (pCO2) in aqueous samples is described. It consists of a gas permeable membrane tube containing a flowing acceptor stream of bromothymol blue indicator solution. Carbon dioxide diffuses across the membrane causing a pH change in the acceptor. This pH change decreases the absorbance of the acid-base indicator which is detected photometrically, with high sensitivity using a multi-reflection photometric detector with an LED light source. Unlike many other common methods used to measure pCO2, this probe has the advantage of not requiring sampling to perform measurements, and avoids potential losses and contamination. This probe has the potential to perform experiments requiring in situ measurements of pCO2, allowing regular measurements of closed system experiments, without removing any of the water column. Compared to indirect methods used to measure pCO2, this probe has the potential to provide more portable and faster measurements. The sensitivity, sampling rate and linear range of the probe can be tuned depending on the required sensitivity and range of measurements, and a measurement rate of at least 36h-1 can be achieved. An application of this probe in real-time analysis of pCO2 flux in a sediment core during a large deposition of organic matter has been described. As a comparison, the measurements of the probe have been plotted against pCO2 calculated from alkalinity using a Gran titration. It is envisaged that the probe could be used for experiments in the laboratory requiring real time in situ measurements, or incorporated into a portable instrument so that field measurements can be easily performed. Although the linear range and sensitivity of this probe can be tuned, the configuration described gave a linear response over the calibration range of 0-5800I?atm pCO2, with a detection limit of 144I?atm. The precision was 1.2 RSD (n=13) at 430I?atm. A? 2011 Elsevier B.V.