Net oxygen exchange between root systems of the sedges Cyperus involucratus Rottb. and Eleocharis sphacelata R.Br. was measured in a bi-compartment apparatus, fitted with a polargraphic oxygen electrode and a platinum wire electrode in the root chamber. The roots of both species consumed oxygen from water in the root chamber with no net exchange when the oxygen partial pressure (ρO2) in the chamber was zero. Rates of oxygen uptakes by roots of intact plants were always lower than excised roots, suggesting a contribution by oxygen transport from the shoots to the root respiratory demand. The contribution of oxygen transported from the shoots increased with diminishing ρO2 in the root medium, approaching the total oxygen demand as ρO2 fell to zero. The roots released oxygen when titanium (III) citrate redox buffer (EH=-350mV) was used in the root chamber to mimic the redox potential of natural sediments. Rates of oxygen release into the reduced solutions were 21 ± 5 and 55 ± 7 µmol O2 hr-1 g-1 root dry weight in the dark (mean values ± 1 standard deviation). These results suggest that an agitated body of water alone is not a suitable medium for measuring root oxygen release by entire root systems. A solution with high oxygen demand is more appropriate.