A procedure for the determination of six natural and synthetic estrogens (diethylstilbestrol, estrone, 17beta-estradiol, mestranol, 17alpha-ethinylestradiol and estriol) in water samples is described. Samples, up to 2000 ml, were concentrated using Oasis HLB solid-phase extraction cartridges. Analytes were derivatized with N-methyl-N-(trimethylsilyl)trifluoroacetamide and determined by GC-MS or GC-MS-MS. The reactivity of several silylation reagents versus aliphatic and aromatic hydroxyl groups contained in the structure of the selected analytes was evaluated. Influence of parameters such as sample pH, nature of the water samples and derivatization conditions on the performance of the whole analytical procedure was systematically studied. Under optimal conditions, quantification limits between 1 and 3 ng/l were achieved for the determination of the considered estrogens in sewage water.