In many countries a multi-barrier concept in a deep geological repository is planned for final disposal of nuclear waste. Many of these different concepts, for example the Swedish KBS-3 model, include an engineered barrier consisting of compacted bentonite. The compacted bentonite barrier will be positioned around copper canisters holding the nuclear waste and act as a transport barrier for various radioactive species. This is due to the compact microstructure as well as the large sorption capacity of compacted water saturated bentonite. During its required lifetime (> 100000 years) a deep geological repository and the bentonite barrier itself will be subjected to changing groundwater composition and flow, geochemistry, temperature, and large doses of ionizing irradiation. The long lifetime and significance of a deep geologic repository mean that it must be regarded as a thermodynamically open system and be assessed in terms of worst case scenarios. In case of water bearing fractures, formed by future movements in the granitic host bedrock, low ionic strength groundwater can endanger the stability of the bentonite barrier. Prolonged erosion of the bentonite barrier might cause significant loss of buffer material and jeopardize its overall functionality. This thesis deals with the microstructure of the water saturated compacted bentonite as a function of compaction, as well as effects of γ-radiation on bentonite dispersions. The microstructure was investigated both indirectly through colloid filtration experiments, as well as using low-angle XRD. Effects of γ-irradiation were investigated in terms of colloid stability and sediment behavior of bentonite (and Na-montmorillonite) dispersions. The colloid filtration experiments, using small gold colloids of different sizes as tracers, shows that the microstructural constraints of bentonite effectively filter even extremely small inorganic colloids. Colloid transport was only observed at very low compactions (i.e. dry density g/cm3) where the average interlayer distances of montmorillonite exceeded the colloid size, indicating interlayer rather than interparticle transport. From the low-angle XRD experiments the free porosity of water saturated compacted bentonite was determined by comparing its basal spacings (interlayer distances + 1 nm) with the hypothetical basal spacings assuming no free porosity. Irrespective of compaction, the maximum free porosity proved to be very low, in line with the colloid filtration experiments. The results showed that existing microstructural models sometimes exaggerate the importance of interparticle voids. The γ-irradiation experiments showed a radiation induced increase in colloid stability. This radiation induced effect also changed the sedimentation behavior of irradiated bentonite dispersions compared to unirradiated samples. The effect is attributed to an increase in surface potential of the bentonite colloids, due to reactions with the short-lived radicals formed upon water radiolysis.