The permeation properties of a semiaromatic polyamide, the poly(m-xylene adipamide) (MXD6), were investigated by water and carbon dioxide permeation experiments (pervaporation and gas permeation tests). Complementary microstructure informations were obtained from calorimetric measurements. Amorphous and semicrystalline MXD6 membranes were studied. The analysis of the water flux through amorphous MXD6 membranes showed a plasticization phenomenon followed by a water-induced crystallization. It resulted that the role played by water in these materials was complex because of the dependence of the water diffusivity on water concentration and time. Because of the presence of crystalline phase, a significant reduction of water and gas permeability of MXD6 and an increase in the delay of diffusion were observed. In terms of barrier properties for water and carbon dioxyde, MXD6 membrane crystallized at high temperature were more performant than water-induced crystallized ones. Correlations between microstructure and transport properties had been so established.