A seal for a valve for use in a pharmaceutical dispensing device, which seal is formed from an elastomeric composition comprising: (a) an isobutylene polymer or co-polymer thereof; (b) a cross-linking agent for the isobutylene polymer or co-polymer thereof; and (c)) an accelerator for the cross-linking agent, wherein the accelerator includes a polysulphide compound derived from a substituted dithiocarbonic acid or derivative thereof.