We performed molecular dynamics simulations on oxygen transport in dry and hydrated (with 3 and 13 wt% water content) amorphous ethylene-vinyl alcohol (EVOH) copolymers. As the water content increases, the intermolecular hydrogen bonding in the matrix decreases and the cavity sizes increase, which result in increased diffusion coefficients. The local chain mobility increases significantly only at 13% water content. The predicted glass transition temperatures fall into reported experimental ranges. The solubility coefficients of oxygen slightly decrease with increasing water content, which indicates that the excessive increase in permeability mainly results from increased diffusion rates. (C) 2004 Published by Elsevier Ltd.