Characterization of mercury contamination in petroleum hydrocarbons (PHs) is necessary in order to assess the risk of corrosion of the processing infrastructure and to assess the level of human exposure to Hg-containing substances. Here we present an accurate and sensitive method for determination of Hg species in PHs by headspace sampling with a possibility of on-line pre-concentration using in-tube extraction (ITEX) combined with gas chromatography-inductively coupled plasma mass spectrometry (GC-ICP-MS) analysis. Mercury species were first extracted from the PHs matrix into an aqueous phase via dithizone chelation and subsequently converted with sodium tetrapropyl borate into volatile derivatives which could be sampled from the headspace prior to GC-ICP-MS analysis. For concentrations in the ng kg(-1) range, the on-line ITEX method was applied, whereas the mu g kg(-1) range was accessible by static headspace. Quantitation of Hg species was carried out by a double isotope dilution method, with quantitative recoveries of methylmercury (MeHg, average 101 +/- 5%) and inorganic mercury (InHg, average 97 +/- 7%) by direct headspace injection. Average recoveries of Hg spikes after on-line ITEX pre-concentration were 95 +/- 3% for MeHg and 98 +/- 8% for InHg. The detection limits for MeHg and InHg were 428 and 46 ng kg(-1) when measured by static headspace, and 2.4 ng kg(-1) and 1.7 ng kg(-1) by on-line ITEX pre-concentration. The accuracy of the pre-concentration method was demonstrated by analysis of a crude oil standard reference material (NIST 2722) certified for InHg.