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Background Ferrous fumarate is useful in iron fortification because of its high bioavailability, mild taste, and relatively low cost. A ferrous fumarate premix for incorporation into salt has been developed by agglomerating ferrous fumarate with appropriate binder materials into salt-size particles followed by microencapsulation.Objective The bioavailability of iron is critical for the usefulness of double-fortified salt. This study examined the in vitrobioavailability of various iron forms in double-fortified salt and microencapsulated ferrous fumarate premixes prepared by various techniques in an effort to identify key processing factors affecting iron bioavailability.Methods Iron in vitrobioavailability was approximated through the rate of dissolution of iron in 0.1 N HCl, which closely approximates the acid in gastric juice. Iron in vivobioavailability was tested using the hemoglobin repletion assay in rats.Results The materials and techniques used in microencapsulating ferrous fumarate had little effect on iron in vivobioavailability: more than 90% of iron in the premixes was released during 2 hours of digestion in the simulated gastric fluid. By incorporating titanium dioxide in the coating materials, the dark reddish-brown color of ferrous fumarate was effectively masked, resulting in acceptable sensory qualities, while maintaining the stability of iodine in the salt. Iron in vivotests in rats have confirmed that the ferrous fumarate microencapsulated in a lipid is highly bioavailable, with a bioavailability of 95% relative to ferrous sulfate.Conclusions These findings were corroborated by field tests in southern India which demonstrated that double-fortified salt containing microencapsulated ferrous fumarate was effective in reducing the prevalence of iron-deficiency anemia and iodine-deficiency disorders.