A process for producing hydrogen gas from a reaction of an organic substance and a base with a recycling of a carbonate or bicarbonate by-product and a regeneration of the base. In one embodiment, reaction of an organic substance and a base produces hydrogen gas and a metal carbonate. The instant invention provides recycling of the metal carbonate by-product. In a preferred embodiment, the metal carbonate by-product is soluble and recycling includes a three step process. In a first step, the soluble metal carbonate is reacted with a metal hydroxide to form a weakly soluble or insoluble metal carbonate that precipitates in a metathesis reaction. The metal hydroxide reactant of the hydrogen producing reaction is also formed in the metathesis reaction and remains in solution. Precipitation of the carbonate thus permits ready isolation of the carbonate by-product, while leaving behind an aqueous metal hydroxide phase that can be returned to and further utilized in the hydrogen producing reaction. The metal carbonate precipitate of the metathesis reaction is thermally decomposed to form a metal oxide solid in a second step. In a third step, the metal oxide is reacted with water to reform the metal hydroxide reactant of the metathesis reaction. The hydrogen producing reaction and recycling process are sustainable in that the metal hydroxide reactant of each reactant is regenerated in the recycling process. In an alternative embodiment, the hydrogen producing reaction produces a metal carbonate precipitate directly and recycling occurs through thermal decomposition of the metal carbonate to form a metal oxide followed by reaction of the metal oxide with water to reform the metal hydroxide employed in the hydrogen producing reaction. In yet another embodiment, a bicarbonate by-product is formed by a hydrogen producing reaction of an organic substance and a base and bicarbonate recovery occurs by heating the bicarbonate to form a carbonate and recycling according to the instant carbonate recycling process.