a barrier spout and cap assembly mounted on a flexible bag or pouch of plastic laminate film having barrier material incorporated therein to prevent the permeation of various gases and especially oxygen . the structures of the spout and its cooperating removable and replaceable cap are such that they provide a barrier to gas permeation at the joint between the spout and bag and through the cap . the cap and associated barrier are removeable and replaceable to facilitate filling of the bag .