Abstract: A sensor for determining the partial pressure of carbon dioxide in a liquid, which comprises a carbon dioxide - permeable, liquid and ion-impermeable membrane enveloping an aqueous medium containing an absorber, a fluorescer and a source of bicarbonate ions, which medium exhibits variations in pH as a function of the partial pressure of dissolved carbon dioxide, the ratio of absorber to fluorescer and the characteristics of the absorption spectrum of the absorber and the excitation and emission spectra of the fluorescer being such that the partial pressure of carbon dioxide in the liquid may be determined from the pH of the aqueous medium as measured by the intensity of the fluorescent emission of the fluorescer.