Through the observation of the pore structure and mercury intruding porosimetry (MIP) experiments of some typicalporous building materials, we found that the diffusion coefficient of the material can be expressed by that of arepresentative elementary volume (REV) in which the pore structure can be simplified as a connection in series of macroand meso pores. Based upon that, a macro-meso two-scale model for predicting the diffusion coefficient of porous buildingmaterials is proposed. In contrast to the traditional porous mass transfer model for determining the diffusion coefficientdescribed in the literature [Blondeau, P., Tiffonnet, A.L., Damian, A., Amiri, O., Molina, J.L., 2003. Assessment ofcontaminant diffusivities in building materials from porosimetry tests. Indoor Air 13, 302-310; Seo, J., Kato, S., Ataka, Y.,Zhu, Q., 2005. Evaluation of effective diffusion coefficient in various building materials and absorbents by mercuryintrusion porosimetry. In Proceedings of the Indoor Air, Beijing, China, pp. 1854-1859, the proposed model relates thevolatile organic compound (VOC) diffusion coefficient of building material not only to the porosity of the buildingmaterial, but also to the pore size distribution and pore connection modes. To verify the model, a series of experiments ofVOC emissions of three types of medium-density board were conducted. The comparison of the model and experimentalresults shows that the proposed model agrees much better with the experimental results than the traditional models in theliterature. More validation for other building materials is needed. The proposed model is useful for predicting the VOCdiffusion coefficient of porous building materials and for developing low VOC emission building materials.