Abstract. Representative elementary volume (REV) has major importance in numerical estimation of effective transport properties of porous materials. The increasing focus on the durability and reliability aspects of cementitious materials, calls for a better understanding of the mass transport phenomenon through their evolving porous microstructure. A multi-scale nature of the cementitious materials imposes a great challenge to modeling efforts. This paper investigates the REV size for numerical modeling of transport in hydrating cement paste. Numerous series of virtual 3D microstructures with different porosities and capillary pore morphologies were generated using Hymostruc platform, a numerical model for cement hydration and microstructure development. Effective diffusion was obtained by using a finite difference scheme. The effect of numerical resolution was also investigated. Based on a statistical analysis, it was concluded that the REV size depends on the complexity of the pore morphology, which further primarily depends on the porosity and employed numerical resolution.