The main drivers of mercury (Hg) compound distribution in seawater are poorly understood, calling for novel spatial and seasonal observations of potential transformations. However, scientific progress is hindered by a lack of intercomparability among incubation studies and the infrequent inclusion of dissolved gaseous mercury species (DGM = elemental Hg (Hg(0)) + dimethyl Hg (DMHg)) despite their importance in the biogeochemical cycle of Hg. We perform a comprehensive quality assessment on our proposed incubation protocol at near ambient concentrations (∼10 × ambient background) including the formation of DGM on three distinct coastal seawaters and discuss intercomparability with previous experimental approaches. We establish an excellent mass balance for tracer isotopes both excluding DGM (199Hg = 99.5% and 201Hg = 100.4%, median) and including DGM (199Hg = 100.3% and 201Hg = 101.7%, median). We find a good median relative standard deviation for experimental triplicates (199Hg(II) ∼ 1.7%, MM201Hg ∼ 1.5%, 199DGM ∼ 10%, 201Hg(II) ∼ 13%, and 201DGM ∼ 22%), enabling the accurate determination of methylation, demethylation, and reduction rate constants at femtomolar concentration levels. We observed Hg(0) formation from MMHg, potentially indicating reductive demethylation. This study highlights the practicability and importance of incorporating DGM species (here, Hg(0), eventually DMHg) in future incubation studies.