The ability of diffusive gas transport and pressurized, convective flow to satisfy internal oxygen demands was examined for an aquatic sedge, Eleocharis sphacelata R. Br. Resistances to convection and diffusion through the plant were quantified from anatomical studies of the airspace dimensions, and these were used in mathematical models to calculate the fluxes required to satisfy oxygen demands measured in the tissue. The greatest resistance to diffusion in the underwater tissue was the submerged culm between the waterline and sediment surface (1560 Ms m−3 per m culm length). Resistances of the nodal intercalary meristem (52 Ms m−3) and rhizome internode (34 Ms m−3) were minor. In contrast, resistances to convection were low in the culms (38 MPa s m−3 per m culm length), and higher in the nodal meristems (93 MPa s m−3). The rhizome internodes had large cortical canals with a low convective resistance (0·75 MPa s m−3), and a parallel spongy pith with a very high resistance (518 MPa s m−3) that is probably short-circuited by convection. The resistance of the submerged culm means that diffusion is inadequate to satisfy oxygen demands in plants growing in >10 cm of water, and that convection is therefore essential in the natural habitat of this species (water to c. 2 m depth). However, a convective oxygen influx as low as 2·8×10−8 mol s−1 per culm (equivalent to a gas flow rate of 3 μl s −1 per culm) could satisfy the entire oxygen demand of the underwater tissue; this value is well below actual rates. At this flow rate, the spongy pith in the rhizome would also remain aerobic: it has a low resistance to diffusion (73 Ms m−3) and could receive sufficient oxygen by diffusion from the node. The data agree well with previous empirical measurements of convection in this species and show that diffusion and convection are both important processes for its aeration.