Poly[bis(phenoxy)phosphazene] and several carboxylated derivatives were examined for use as membranes to separate tritiated water and deuterated water from light water. The polyphosphazenes were solvent cast from dioxane onto a support material of polyethylene terephthalate. These membranes were tested with water containing 10 pCi/ml tritiated water or 10 wt% deuterated water under dead-end filtration conditions. Poly[bis(phenoxy)phosphazene] provided separation of tritiated water up to 31% within the temperature range of 4 to 23 degrees C. The 5 to 10% carboxylated-polyphosphazene membrane shows a tritiated water separation of 43% at 4 degrees C, but the separation approaches zero above approximately 10 degrees C. Membranes with higher carboxyl content (20 and 30%) were much less efficient for tritiated water separation. Under the same low temperature conditions, and with both types of polyphosphazene membranes, deuterated water separation was considerably less efficient.