A polymer electrolyte membrane or gas diffusion electrode includes an ion-conducting polymeric material which includes moieties of formula (A) which are substituted on average with more than 1 and 3 or fewer groups (e.g. sulphonate groups) which provide ion-exchange sites and hydrogen atoms of said moieties are optionally substituted, wherein each X in said moieties of formula A independently represent an oxygen or sulphur atom. The ion conducting polymeric material is suitably prepared by controllably sulphonating a polymeric material using about 100% sulphuric acid at 34° C. to 36° C.