Adsorption of bisphenol-A (CAS 85-05-7), 17 beta-estradiole (CAS 50-28-2) and 17 alpha-ethinylestradiole (CAS 57-63-6) to activated and to inactivated sludge from wastewater treatment plants (WWTPs) was investigated, thus allowing to distinguish between pure adsorption and biosorption. For the investigated substances the determination of the adsorption kinetics is based on experiments performed according to the OECD guideline 106 and on free concentration measurements in the liquid phase. The description of the adsorption behaviour occurred via Freundlich Adsorption Isotherms. Additionally specific adsorption coefficients KD, KOM and KOC were calculated. The results of these calculations were compared to KOC values obtained with a HPLC method according to the OECD guideline 121. All substances showed a high adsorption affinity to the adsorbent and in spite of the application of very high initial concentrations no saturation level could be reached. Within a contact time of 24 h, no difference between the adsorption to activated and inactivated sludge could be detected. The calculated KD values were within a range of about KD = 1000 l kg(-1) for the investigated compounds and showed a clear concentration dependency in the case of bisphenol-A. Adsorption was also found to depend on pH. The experimentally determined KOC values of the investigated substances were significantly higher than the results obtained with the HPLC method described in OECD guideline 121.