Our study shows that magnesium ions which have so far been considered as non-gelling ions for alginate do induce alginate gelation. Rheology is used to examine effects of alginate chemical composition as well as alginate and magnesium ion concentration. Gelation in this system occurs at ca. 5-10 times higher concentration of ions than reported for calcium-based gels. Alginate network formation with magnesium ions is very slow and is typically accomplished within 2-3 hours. Gelation with magnesium ions is also strongly dependent on alginate chemical composition as the presence of long guluronic units privileges faster gel formation.