TECHNOLOGY FOCUS - POLYMERS - Preferred Membrane: The units of formula (I), (II) and/or (III) are functionalized to provide ion exchange sites. The polymer is sulphonated. The membrane comprises 45-100 mole% units of formula (I), and 0-55 mole% units of formula (II) and (III). The polymer is a homopolymer having a repeat unit of formula (VII)-(X).-((E1-(Ar')-(phi)m'-E'')A'-(phi-C(O)-(phi)w'-G'-((phi)r'-C(O)-phi)s')B')- (VII)-((E1-(Ar')-(phi)m'-E'')c'-(phi-SO2-(phi)z'-G'-((phi)t'-SO2-phi)v')D)- (VIII)-((phi-C(O)-(phi)w'-G'-((phi)r'-C(O)-phi)s')B'-(E1-(Ar')-(phi)m'-E'')A')- (IX)-((phi-SO2-(phi)z'-G'-((phi)t'-SO2-phi)v')D'-(E1-(Ar')-(phi)m'-E'')C')- (X)G' = direct bondw' = 0 or 1m' = 0 or 1s' = 0A' = 0 or 1B' = 0 or 1Ar' = phenyl, diphenyl carbonyl, diphenyl sulfonyl or naphthylE1 = direct bond or OC' = 0 or 1z' = 0 or 1v' = 0D' = 0 or 1The polymer is crystalline and includes at least ketone or biphenylene (preferably O-biphenylene-O, especially O-naphthalene-O) and has an inherent viscosity at least 0.3 and glass transition temperature Tg at least 154 degrees C (at least 144 degrees C).Preferred Process: The sulfonation is carried out in concentrated sulfuric acid. EXAMPLE - 4,4'-dichlorodiphenyl sulfone (104.25 g), 4,4'-dihydroxydiphenyl sulfone (6.75 g), 4,4'-dihydroxybiphenyl (16.74 g) and diphenyl sulfone (245 g) were purged with nitrogen for 1 hour at 140-145 degrees C to form colorless solution. Potassium carbonate (50.76 g) was then added and temperature was raised to 180 degrees C for 30 minutes, 205 degrees C for 1 hour, 225 degrees C for 2 hours, 265 degrees C for 30 minutes and 280 degrees C for 2 hours. The mixture was then cooled and dried in an air oven at 120 degrees C.    The polymer (15% wt/v) was dissolved in N-methylpyrrolidone. The homogeneous solution was cast onto clean glass plates and then drawn down to give 300 micrometers films. Evaporation at 100 degrees C for 24 hours produced membranes of mean thickness 40 micrometers. The membrane was then immersed in deionized water (500 ml) for 3 days and dried in an oven at 50 degrees C for 1 day. The water uptake was found to be 30.7%.