Abstract This paper summarises the work undertaken to characterise the physico-chemical properties of alginate gel beads in simulated gastro-intestinal (GI) conditions. Two types of alginate beads were investigated: “strong” (strongly gelled) and “weak” (weakly gelled) beads prepared by long and short exposure to a calcium chloride gelling bath, respectively. The beads were found to shrink in gastric conditions and swell in intestinal conditions due to changes in electrostatic forces in the gel matrix at the different pH and ionic strength conditions. We found a good correlation between the NMR transverse relaxation time ( T 2) of the water protons within the gel and alginate concentration which was dependant on environmental conditions. T 2 shortening was observed in gastric conditions reflecting the formation of a more dense gel network on shrinking. In intestinal conditions, T 2 increased reflecting the formation of a more open, porous gel network on swelling. This was corroborated by electron microscopy which clearly depicted the changes in gel density in simulated GI conditions. The mechanical properties of the beads similarly reflected the changes in the gel microstructure with the beads becoming stronger in gastric conditions and weaker in intestinal conditions, respectively. The beads were shown to eventually disintegrate towards the end of the intestinal phase which may make these alginate gel beads an attractive option as controlled delivery devices in the gastro-intestinal tract. The in vivo behaviour of the beads within the GI tract is investigated using non-invasive magnetic resonance imaging in the second paper of this series.