An insert (8) for a screw (1) or crown (1′) cap for the closure of bottles (10) is described, the said cap (1, 1′) including a body (2) and the insert (8) being designed to be fixed to the body facing the interior of the bottle (10) when the cap (1, 1′) is closed over the said bottle. The insert (8) comprises a sealing element (9) capable of being compressed in one part between the body and a portion of the bottle (10) when the cap (1, 1′) is closed over the bottle, as well as a permeating element (16, 109, 209), connected to the sealing element, impermeable to liquids and having a permeability to oxygen measured at 20° C. of between 10−6 and 10−10 (Ncm3*cm/cm2*cmHg*s), which is designed to close a passage made in the said cap between the inside and outside of the bottle, in order to control the flow of oxygen between the inside and outside of the bottle.