N,O-bis(trimethylsily)trifluoroacetamide　(BSTFA)　and　N-methyl-N(trimethylsily)　trifluoroacetamide　(MSTFA)　are　common　derivatization　reagents　used　in　the　GC-MS　analysis　of　estrogen　steroids　such　as　estrone　(E1)　and　17　alpha-ethinylestradiol　(EE2).　In　this　study,　three　trimethylsilyl　(TMS)　steroid　derivatives,　mono-　and　di-trimethylsilyl　EE2　and　mono-trimethylsilyl　E1,　were　observed　during　the　derivatization　of　EE2　with　BSTFA　or　MSTFA　and/or　GC　separation.　Factors　influencing　the　production　of　multiple　TMS　derivatives　and　their　relative　abundance　were　examined.　It　was　found　that　both　methanol　and　bisphenol　A　competed　with　estrogenic　esteroids　when　reacting　with　silylation　reagents,　and　thus　affected　the　formation　of　TMS　derivatives　and　their　relative　abundance　in　the　derivatization　products.　Methanol　was　found　to　be　more　reactive　than　bisphenol　A　with　the　BSTFA　reagent.　None　of　the　three　solvents　tested　in　this　study　could　prevent　the　generation　of　multiple　TMS　derivatives　during　the　derivatization　of　EE2　with　BSTFA,　followed　by　GC　analysis.　A　similar　result　was　observed　using　MSTFA　as　the　derivative　reagent　followed　by　GC　analysis.　Thus,　the　suitability　of　BSTFA　or　MSTFA　as　the　derivatization　reagent　for　the　determination　of　El　and　EE2　by　GC-MS,　under　the　conditions　reported　here,　is　questionable.　This　problem　can　be　solved　by　adding　trimethylsilylimidaz　(TMSI)　in　the　BSTFA　reagent　as　recommended,　and　the　performance　of　the　method　has　been　proved　in　this　study.