A thermoplastic essentially oxygen-impermeable plasticized sealant is formed by melt-blending a hydrogenated styrene-conjugated diene-styrene (SMS) block copolymer rubber with a polystyrene-polyisobutylene-polystyrene (SIBS) block copolymer rubber and a polyolefin plastic with a liquid polyisobutylene (PIB) oil plasticizer provided the PIB oil is present (i) in the range from 5% to less than 50% by weight of the sealant and (ii) in relation to total rubber, in the range from 0.3 to 0.8. The required minor amount of PIB oil, relative to total rubber, in combination with SIBS present in a minor amount relative to the amount of plasticized sealant, provides the sealant with unexpectedly better oxygen barrier properties and load bearing at 82° C. (180 ° F.) than a comparable blend of SIBS with mineral oil; the sealant is also essentially free of tack, adhesive properties and oil-bleed, with essentially no detackifier present. The PIB-oil plasticized SIBS sealant has a haze of less than 15%; a composite made by melt-bonding a core layer between polyC 2 -C 3 olefin sheets, maintains a haze less than 14%. The plasticised sealant is particularly useful for sealing elements for containers in which foods, beverages and medical products must be preserved for a long period.