Alginate dialdehyde (ADA) biopolymers possessing a degree of oxidation of either 10 or 50% (10-ADA and 50-ADA, respectively) were characterized using FTIR, XPS and SEC. The aldehyde vibrational mode at 1740 cm⁻¹ was observed only in ADA which had been equilibrated under ambient conditions while dry samples did not display this band. Spectral changes, both FTIR and XPS, were consistent with formation of hemiacetal moieties. The two types of ADA (10-ADA and 50-ADA) were used as macromolecular crosslinkers to form labile covalent crosslinks in alginate hydrogels. The compositions and properties of the hydrogels were explored through measurement of water uptake and stability in aqueous solution, and characterising the internal structure and mechanical properties by cryogenic scanning electron microscopy and tensile testing, respectively. A decrease in water content was observed when using 50-ADA as compared to 10-ADA correlating with a higher number of crosslinks formed in the hydrogel incorporating 50-ADA. Water uptake also correlated with the amount of 50-ADA incorporated. All ADA–alginate hydrogels displayed short term stability of 3 days after immersion in aqueous solution. The stability of the 50-ADA containing hydrogel was enhanced by introducing ionic crosslinking. Tensile properties of the hydrogels were found to be dependent on the overall polymer density and uniformity of the crosslinking.