An analytical method for simultaneous in situ ethylation, of organolead, organotin and organomercury compounds in aqueous samples was developed using a new derivatisation agent, bromomagnesium tetraethylborate (BrMgEt(4)B). The determination of lead, tin and mercury compounds was done by species-specific isotope dilution, derivatisation and GC-inductively coupled plasma MS (GC-ICP-MS) or by GC-MS. The recovery and accuracy of the derivatisation were evaluated. The effect of pH and the relative quantity of derivatisation agent were studied.