Seven sex hormones (estradiol, ethinylestradiol, estrone, estriol testosterone, progesterone, and androstenedione) were spiked into a synthetically prepared wastewater. There, it was found that the sum of the aqueous and solid phase concentrations of the four estrogens did not add up to their theoretical concentration in the influent and that these concentrations decreased over time in the chilled feed. The remaining hormones present in the feed did not exhibit such behavior. We are conducting studies to clarify the mechanism causing the non closure of the mass balance for the four compounds cited above in the absence of biological activity, as well as to identify the causing agent. The two possible causes considered are irreversible adsorption onto the solid components (such as clay and rabbit food) of the feed and/or abiotic reactions that degrade estrogenic compounds over time. The concentration of steroids is measured in the aqueous and solid phases of the synthetic wastewater separately; for this purpose a new solids extraction method is being developed.