In order to successfully participate in intercalibrations, the so-called Proficiency Tests, the CWA Analysis Group must keep work up and analysis methods for all chemicals on the OPCW Schedule 1 List. In this report a method for verification of lewisite 1 and 2 (or their corresponding products of hydrolysis) is described where a derivatisation reagent is used to chemically modify the substance prior to GD/MS analysis. Four different thiols have been tested as possible derivatisation reagents; ethanethiol, ethanedithiol, 3,4.dimercjaptotoluene and methyl thioglycolate (thioglycolic acid methylester) of which the last one was found to be the most suitable. Work up procedures for lewisite in water and soil hae been studied and as suitable method derivatisation in acidified water followed by hexane extraction was chosen. The stability in storage of lewisite and its corresponding products of hydrolysis in water and soil have been studied and show that the hydrolyte products of lewisite 1 and 2 are possible to find after one week of sample storage.