The diffusion coefficient is a highly important parameter either as a durability indicator in the field of civil engineering or as an assessment criterion in the domain of nuclear waste disposal. In this paper we address the fundamental issue of the diffusion coefficient evolution with the water saturation level of cement-based materials. The study is comparing concretes and cement pastes cast with either blended cement or Portland cement. The analysis emphasizes the impact of the pore size distribution on the diffusivity as a function of the saturation degree. (C) 2013 Elsevier Ltd. All rights reserved.