Carboxylated poly(diaryloxy)phosphazene was examined as the active constituent of the composite membranes to separate tritiated water (HTO) from light water. These membranes were tested with wafer containing 10 800 pCi/l and 3 mu Ci/l of tritiated water, respectively, under cross-flow filtration conditions. Reductions in the permeate of nearly 30% HTO were observed with these tritium concentrations. Low temperature (5 degrees C), low pressure (137.9-551.6 kPa), and high pH (near 10 or above) were required to obtain such reductions (rejection). Salt species (Na2SO4, CaCl2 and CaSO4) at various concentrations and pressures, within a 137.9-551.6 kPa range, did not appear to affect the HTO separation efficiency. Mass balances performed during these experiments indicate an unaccounted small amount of tritium (0.5-2.2%). Sorption experiments with the polyphosphazene suggest that the membrane does not operate by an ion exchange mechanism; that is, tritium accumulation within the membrane.