A method for detecting mustard gas degradation products thiodiglycol (TDG) and thiodiglycol sulfoxide (TDGO) in water and sediment samples using gas chromatography-tandem mass spectrometry (GC-MS/MS) after derivatization with 1-(trifluoroacetyl)imidazole (TFAI) was described. Selected reaction monitoring mode (SRM) of tandem mass spectrometry was developed for analysis of TDG and TDGO derivatives while analysis by gas chromatography-atomic emission detector (GC-AED) was performed using the 181 nm sulfur canal. TFAI derivatization conditions were optimized and the method validated. Two derivatization agents were compared, TFAI and N,O-bis(trimethylsilyl) trifluoroacetamide (BSTFA), where derivatization using TFAI occurred quicker and under milder conditions than using BSTFA. Water samples containing TDG and TDGO were evaporated to dryness under nitrogen, dissolved in organic solvent followed by reaction with TFAI. The limit of detection (LOD) for TDG and TDGO were 0.01 and 5 ng/mL, respectively. The limit of detection for TDG was decreased by two orders of magnitude if TFAI is used for derivatization rather than silyl derivatizing agents (e.g., BSTFA). TFAI has three major advantages in comparison to BSTFA, the first is much higher sensitvity, the second is a very clean background of chromatograms, and the last one is very mild conditions of derivatization. Moreover, by-products are not formed during derivatization of TDGO by TFAI in comparison to derivatization by silylating agents.