Particulate HgS is generally thought to be the most abundant Hg species in stored petroleum hydrocarbons stocked on-shore. It is also assumed that due to its high stability constant, HgS is chemically inert. However, results from the current study would suggest otherwise. First, a comparison study of the analytical performance of ICP-MS and CV-AFS with respect to matrix effects showed no significant difference in the measured Hg concentrations in studied samples when CV-AFS is used in the alkali mode. Subsequently, the suitability of three quantification methods during Hg speciation was investigated. Both external calibration and standard addition methods proved troublesome with the former showing matrix dependence and the latter being hindered by the formation of an emulsion during the derivatization step. Results from species-specific isotope dilution (SS-ID-GC-ICP-MS), on the other hand, obtained at different equilibration times, showed a random variation in the calculated Hg2+ concentration (RSD 32%), suggesting that factors independent of equilibration time cause the observed variation. Further sedimentation of Hg particulate by means of ultracentrifugation improved the precision of SS-ID-GC-ICP-MS by 10-fold. These results would suggest that Hg particles in petroleum products are reactive at low temperature during Grignard alkylation.