Opalinus clay has a high sealing capacity and is therefore considered as a viable candidate for hosting high-level radioactive waste. Assessment of the long-term containment function of clays requires understanding and modelling of mass transport through evolving pore systems. Development of pore network models for diffusion, which can be coupled to models for deformation and micro-cracking, is reported. Effects of clay texture and solid phase constitution are calculated and analysed. The results are in the range of, but slightly over-predicting, experimentally measured coefficients of diffusion in different clay directions. Further model improvements require better knowledge of micro-pore tortuosity, which awaits higher resolution experimental techniques.