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With the utilisation of recycled aggregate concrete (RAC) in building construction, the durability of this material requires particular attention, especially in terms of chloride penetration. This paper presents a numerical study on the chloride diffusion mechanism within RAC. Considering the random distribution of recycled coarse aggregates (RCA), a five-phase RAC model - including new mortar, adherent old mortar, new interfacial transition zone (ITZ), old ITZ and original natural coarse aggregates - is proposed to predict the effective diffusion coefficient of chlorides in RAC. The parametric studies, based on a series of critical factors (i.e. volume fraction of RCA, adhesive ratio of old mortar, chloride diffusivity of adherent old mortar, thicknesses of old and new ITZs and chloride diffusivity of old and new ITZs), reveal the properties of each phase and their individual impact on the effective diffusion coefficient of chlorides in RAC. The results obtained indicate that, interestingly, the effective diffusion coefficient tends to vary in terms of its relationships with high-quality adherent old mortar, lower adhesive ratio of old mortar, smaller thickness of ITZs and relatively superior chloride penetration resistance of ITZs, which cannot be found from existing models and experiments. [ABSTRACT FROM AUTHOR]