Calix[4]
arene ± resorcin[4]arene (2:1) receptor adsorbate 2 equipped with four didecyl sulfide groups selfassembles in monolayers (SAMs) on gold. These monolayers were characterized by contact angle measurements, polarized infrared external reflectance spectroscopy (PIERS), electrochemical capacitance and resistance measurements, and X-ray photoelectron spectroscopy (XPS). Interactions of a monolayer of 2 with steroid guests were investigated by surface plasmon reso-nance (SPR) spectroscopy. The results show that steroids interact more strongly with a monolayer of 2 than with a hydrophobic reference monolayer of octadecanethiol. There is virtually no detectable interaction with a hydrophilic reference monolayer of 11-mercaptoun-decan-1-ol. Mixed monolayers of 2/11mercaptoundecan-1-ol and of 2/decanethiol showed lower SPR responses with the steroids than the pure monolayer of 2. Concentration-dependent experiments with prednisolone-21-acetate, corticosterone-21-acetate, and cortisone-21-acetate on the mixed monolayer showed Langmuir-type adsorption with affinity constants between 2.0 Â 10 5 and 3.5 Â 10 5 m À1 .