Sodium hydrosulfide (NaSH), which is utilized for recovering rare metals, was synthesized in a laboratory-scale batch reactor by absorbing a simulated flue gas containing H2S into NaOH solution. The effects of H2S flow rate, NaOH concentration, and reaction time on the synthesis of NaSH were examined. With an increase in the H2S flow rate, the absorption ratio, conversion ratio, and total NaSH productivity showed a decreasing tendency. On the other hand, a higher concentration of NaSH could be synthesized with a higher concentration of NaOH. Most of the Na2S (the intermediate product) were produced at a pH>12, and the NaSH synthesis reaction was feasible at a pH 11.5. Contrary to the increased H2S flow rate, increased NaOH concentration resulted in an enhanced Na2S/NaSH ratio. With a maximum equivalent ratio of NaOH/H2S at 0.88, the chemical composition of the product could maintain equilibrium with the highest NaSH concentration and less than 1% weight-to-weight ratio (w/w) Na2S concentration. © 2015 American Society of Civil Engineers.