A polymer with a rigid, randomly contorted molecular structure (see Figure), incorporating fused rings connected by spiro-centres, may be precipitated or cast from solution to give microporous powders and membranes stable up to temperatures of 350degreesC, with apparent surface areas > 600 m(2) g(-1). Organophilic membranes may be formed, as demonstrated by the separation of phenol from water by pervaporation.