A gas chromatography/mass spectrometry method for the simultaneous quantitative determination of natural and synthetic estrogens (17β-estradiol, estrone, 17α-ethinylestradiol, and mestranol), phytoestrogens (genistein and β-sitosterol), and xenoestrogens (benzyl butyl phthalate, dibutyl phthalate, bisphenol A, 4-nonylphenol [NP], 4-nonylphenoxyacetic acid [NP1EC], 4-nonyl-phenol diethoxylate [NP2EO], and α-endosulfan) in effluents of sewage treatment plants (STPs) was developed. Identification and quantification were carried out with the standard addition method using analyte-specific and, in some cases, deuterium-labeled internal standards. The effluents of 18 STPs were investigated. Apart from α-endosulfan and mestranol, all selected substances were detected in the majority of samples. The median concentrations of steroidal estrogens were between 0.4 ng/L (17α-ethiny-lestradiol) and 1.6 ng/L (17β-estradiol). The metabolites of the nonylphenol polyethoxylates, NP, NP1EC, and NP2EO were found in concentrations ranging from the upper-ng/L-range (NP) to the lower-μg/L range (NP1EC). For all substances except mestranol and α-endosulfan, median values were calculated and compared to the results of other investigations in Europe and the United States. Possible dependencies of measured concentrations on the geographical location, the capacity, the influent composition, and the technical fitting of the STPs are discussed.