Cyperus papyrus L. (papyrus) is an emergent wetland species with C 4 photosynthesis. Culms of papyrus possess numerous large intercellular air cavities and functional ‘Kranz’ chlorenchyma which are involved in CO 2 recycling in the culm. In darkness, the CO 2 concentration in the culms increased to 74 times that of the ambient air. In the light, the culms greatly reduce the intercellular CO 2 concentrations by internal CO 2 recycling via photosynthesis. Results suggest that 35–57% of the CO 2 respired by the culm pith and rhizomes may be refixed by culm photosynthesis. The dynamics of O 2 transport in the intercellular spaces of the culms and the rhizomes were also studied. Both illumination and prolonged darkness had significant effects on the O 2 concentrations in the culm and rhizomes. While the water surrounding the rhizomes remained strongly hypoxic, the O 2 concentration in the submerged rhizomes was 15.1% during the day and 10.3% at night. The diffusive fluxes of CO 2 and O 2 within the papyrus plant during the day and night were calculated. Results suggest that rapid CO 2 exchange occurs between the ambient air, internal atmosphere and the culm photosynthetic tissue. Also, there is a high O 2 flux, particularly at night, which is generated in the intercellular air spaces between the culm and the rhizome.