Dehydration of water/1-1-dimethylhydrazine (UDMH) mixtures using hydroxysodalite (HS) zeolite membranes is presented and its performance during separation of water/UDMH mixtures was evaluated. HS zeolite was coated on an external surface of porous supports by hydrothermal synthesis. The tubular supports were made of mullite. Water/UDMH mixtures were separated at ambient temperature and atmospheric pressure by pervaporation using the HS zeolite membranes. The membranes showed very high selectivity of water for all water/UDMH mixtures. A separation factor as high as 52,000 was obtained for the UDMH feed concentration of 5%. Total mass flux was also obtained in a range of 0.20-2.40 kg/m h. The results confirm the superior performance of the HS zeolite membranes for dehydration of water/UDMH mixtures.