This paper discusses a new silylating reagent - dimethyl(3,3,3-trifluoropropyl)silyldiethylamine (DIMETRIS) - for the derivatisation of non-steroidal anti-inflammatory drugs (NSAIDs) prior to GC-MS analysis. DIMETRIS reacts with seven target compounds (diclofenac, ibuprofen, ketoprofen, naproxen, flurbiprofen and paracetamol, as well as salicylic acid, a degradation product of acetylsalicylic acid) at 30°C for 30min, producing mono-O-dimethyltrifluoropropylsilyl (mono-O-DMTFPS) derivatives. The mass spectra of these new derivatives are given and fully interpreted. The usefulness of mono-O-DMTFPS derivatives for the qualitative and quantitative analysis of NSAIDs using GC-MS is compared with that of trimethylsilyl and methyl analogues. Methyl derivatives are found to be less appropriate for this purpose because they yield the lowest detector responses during GC-MS measurements. Both silyl derivatives are more suitable for determining such NSAIDs, although the matrix effect with mono-O-DMTFPS derivatives is smaller. Finally, SPE-GC-MS(SIM) based on the derivatisation of NSAIDs by DIMETRIS is evaluated, validated and applied to the determination of these drugs in sea water (Baltic Sea) and wastewater samples collected in Poland. This work confirms that DIMETRIS is suitable for the trace analysis of NSAIDs in real samples and provides an interesting alternative to silylating and methylating reagents.