Abstract : The objective of this study was to modify and optimize US Army Element, Assembled Chemical Weapons Alternatives (USAE ACWA) method BGCAPP-204 (VX in Caustic Hydrolysate by Cool on-Column GC/MS) for analysis of ethyl S-2-diisopropylaminoethyl methylphosphonothiolate (VX) in acidified VX hydrolysate matrices. The modified procedures included the injection of VX spikes directly to the acidified hydrolysate samples and the use of an alkaline buffer (pH approximately 12) to adjust the samples to a pH close to 11 prior to VX extraction. The applicability of the modified method was tested and successfully demonstrated for VX hydrolysate alone and in the presence of energetics hydrolysate over a sample pH range of 5 to 9. VX extraction recoveries were -80%. No VX reformation was observed over the pH range studied.