Ground granulated blast-furnace slag (ggbfs)/OPC blends are among the materials being considered as possible encapsulating matrices for low and intermediate level radioactive wastes. It is therefore desirable to identify any microstructural modifications which may result in these systems frmn exposure to ionizing radiations. In order to investigate this, ggbfs/COC blends were γ-irradiated to >80M3y at 50°C over a period of 2 years. These were examined by electron optical, X-ray and thermal analysis techniques and the results ccmpared with controls subjected to a similar hydration regime. The morphological and ccmpositional similarities and differences revealed between the irradiated and control samples are presented and discussed.