An analytical approach involving aqueous-phase derivatization followed by gas chromatography/ion trap mass spectrometry (GC/ITMS) for the determination of methylmercury in waters is described. The aqueous-phase derivatization is based on the formation of a more hydrophobic compound between MeHg+and BPh−4, i.e. MeHgPh, which is extractable into dichloromethane. The detection limit of the method for MeHg is 1 ng/mL for Hg when 100 ml water was analyzed. Recoveries from standard addition to tap water, lake water are 96 ± 8% and 92 ± 8%, respectively.