Membranes frequently contain built-in 'fixed' charged sites (e.g. immobile –(SO₃⁻) in polystyrenesulfonate), or intentionally added, hydrophobically trapped, 'mobile' sites [e.g. tetraphenylborate in plasticized poly(vinyl chloride)]. Such membranes with charged sites are named sited membranes. Ions of opposite sign in the @MT06878@ are '@C01371@'. Ions of the same sign as sites are not present in significant quantities, and are @C01141@. Sited membranes are 'permselective' to @C01371@ i.e. only @C01371@ exchange into the @MT06878@ and therefore have some @M03955@ in the @MT06878@ bulk.