Supercritical carbon dioxide in water emulsion-templated synthesis of porous alginate hydrogels with a narrow range of macropore sizes was investigated. Hydrogels are three-dimensional (3D) polymeric networks capable of absorbing large quantities of water and are widely used in the biomedical fields as drug-delivery devices, wound dressings and scaffolds for tissue-engineering. The CO2 was used as a reagent increasing the acidity of aqueous phase and a templating agent. Results show that the reactive emulsion templating (RET) hydrogels were reversible under certain conditions, when left exposed to the atmosphere the CO2 escapes from the hydrogels and the alginate structure destabilizes within 48 hours.