Concrete　is　rarely　saturated.　Reliable　durability　design　of　marine　concrete　structures　requires　a　solid　understanding　of　the　long-term　chloride　transport　in　unsaturated　concretes.　This　paper　presents　a　critical　analysis　of　the　time-dependent　chloride　diffusion　coefficient　in　unsaturated　cementitious　materials　exposed　to　marine　environment.　Evolutions　of　pore　structure　and　chloride　diffusion　coefficient　in　saturated　cementitious　materials,　along　with　the　role　of　the　degree　of　water　saturation　in　long-term　chloride　diffusion,　are　analyzed.　It　is　emphasized　that　the　long-term　sharp　decrease　of　the　chloride　diffusion　coefficient　in　marine　cementitious　materials　is　not　primarily　caused　by　densification　of　the　microstructure　due　to　hydration,　but　by　the　decreasing　degree　of　water　saturation　with　depth　in　the　surface　part　of　the　materials.　The　effects　of　water/binder　ratio　and　supplementary　cementitious　materials　on　chloride　diffusion　coefficient　are　different　between　saturated　and　unsaturated　cementitious　materials.　(C)　2019　Elsevier　Ltd.　All　rights　reserved.