By providing a synthetic closure which comprises at least one thermoplastic polymer and, as an additive, at least one fatty acid derivative, in particular a fatty acid ester or a fatty acid amide such as a stearamide, a synthetic closure is attained which achieved substantially enhanced properties. In particular, it has been found that the oxygen transfer rate of the closure is reduced substantially, thus reducing unwanted oxidation of wine. Furthermore, the use of a fatty acid derivative additive improves the performance characteristics of synthetic corks, such as extraction force, ovality control, diameter control and length control.