The reaction of Hg 2+ with sulfite is a major identified reduction pathway in the atmosphere. UV absorption spectroscopy was used to study the kinetics of Hg 2+ reduction by sulfite (Na 2 SO 3 ) in the presence of fly ash. Upon the addition of Cumberland and Shawnee fly ash samples, the reduction rates were 0.0071 ± 0.0008 and 0.0009 ± 0.0006 s −1 , respectively. This represents c.a. 40 and 90 % decreases in the homogeneous rate, 0.013 ± 0.007 s −1 . The reduction reaction was also observed when Cumberland was added without Na 2 SO 3 . Sulfur elemental analyzer and high-resolution field emission scanning microscopy with energy dispersive X-ray spectroscopy (HR-FE-SEM-EDS) characterization confirmed that Cumberland fly ash particles were rich in sulfur. Nanoparticle Tracking Analysis (NTA) determined the mean particle size in solution to be 246 ± 25 nm for Cumberland fly ash and 198 ± 14 nm for Shawnee. To obtain further insight on observed Hg 2+ homogeneous reduction rates by sulfite, the effects of several environmental variables were investigated. Hg(NO 3 ) 2 and HgO were used as the sources of Hg 2+ . Extended pH (1–7) and temperature (1.0–45.0 °C) ranges were studied for the first time. The enthalpies of activation for the HgO reduction were 94 ± 3 kJ mol −1 at pH 1 and 92 ± 4 kJ mol −1 at pH 3, while the entropies were 33 ± 9 J mol −1 K −1 at pH 1 and 30 ± 10 J mol −1 K −1 at pH 3. It was determined that increasing ionic strength, especially with nitrate species, slows down the reaction at pH = 7. Significance of the results on the variability of mercury reduction by sulfite at various environmental conditions, and its implication in modelling are discussed.