Sodium birnessite ((Na2Mn2Mn5O13)-Mn-II-O-IV. H2O), a layered manganese(IV) oxide-based phase, gives a liquid polysulfide cure that is too rapid for normal application when added at 10 pph polysulfide. The curing behavior of sodium birnessite added as 5 pph, 4 pph, and as a 5:5 pph mixture with an inert natural manganese dioxide was compared with that of a readily available manganese-based commercial curing agent. The rate of cure at 5 and 4 pph was slower than the commercial agent at 10 pph and led to products with lower tensile strength. The cure with the 5:5 pph mixture gave a more rapid reproducible cure than that of the commercial agent, making a product with a higher tensile strength and lower elongation, which indicates better curing and higher crosslinking. The improved performance of sodium birnessite as a curing agent is consistent with the presence of Mn2+ in the lattice, creating vacancies in the Mn4+-O2- lattice and increasing the mobility of Mn4+ and its transport to the surface of the solid to oxidize the polysulfide.