In order to better define the cycling of mercury in the global troposphere, we have studied over the past four years the homogeneous gas-phase reaction of elemental mercury (Hg 0) with hydrogen peroxide (H 2O 2), 0 3, methyl iodide (CH 3I), and dimethyl sulfide ((CH 3) 2S or DMS) in FEP Teflon® reactors. For the first time, we report laboratory measurements of the reaction rate constant of Hg 0 with H 20 2. Results which were at or below our detection limit suggest this reaction must have a rate constant of <8.5 x 10 −19 cm 3 molecule −1 s −1. With 8.5 x 10 −19 cm 3 molecule −1 s −1 as an upper limit rate constant, mean H 20 2 concentrations of 1.0 ppb would yield a global average Hg 0 residence time of 1.5 years, similar to that calculated by reaction with 30 ppb 0 3. Methyl iodide or (CH 3) 2S are not geochemically important in the cycling of gaseous Hg 0. New research should focus on both the homogeneous and heterogeneous reactions of Hg 0 with Cl 2 and other reactive CI species, OH, HO 2 and organic peroxyradicals (RO 2).