AbstractCellulose acetate (CA) and poly(bromophenylene oxide, dimethylphosphonate) (PPOBrP), which are compatible polymers, have been cast from solution to give both dense and asymmetric alloy membranes. Membranes containing PPOBrP with different degrees of phosphonylation have been prepared. The water absorption of these membranes increases with the number of phosphonate ester groups, but is kept in the range of 12–16 wt‐ water for most of the alloy compositions, which contained 20–80 wt‐ PPOBrP. The morphologies of asymmetric membranes obtained from various casting formulations were studied by scanning electron microscopy. Two different structures were identified: (1) the well‐known dense skin resting on an open‐celled foam, and (2) skin resting on a porous layer which displays a two‐phase morphology. In the latter, dense spheres (0.1–1 μm) appear to grow out of a continuous polymer network. The membranes have been tested for hydraulic permeability and separation of water from salt solutions by reverse osmosis. In general, the asymmetric alloy membranes that had been annealed at 90–95°C display salt rejections>90 and water permeation rates of 10–30 gfd. Since the phenyl ring of the PPOBrP component was brominated prior to membrane fabrication, the membranes exhibit exceptional tolerance to chlorinated water (20–80 ppm), as demonstrated in short‐time durability tests. The irreversible collapse of these membranes occurs at applied hydraulic pressures far above 1200 psi. A cross linking between the two polymer components in the membranes and some suggestions for further improvement of these me
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