The adsorption of proteoheparan sulfate, a strongly negatively charged proteoglycan, at (hydrophobic) methylated silica surfaces was investigated with in situ ellipsometry. In particular, the effects of electrolytes in the physiological concentration ranges were studied. Both general electrostatic and cation-specific ion-binding effects were shown to be important for the interfacial behavior of this macromolecule. While Ca2+ causes an increase in the adsorbed amount of proteoheparan sulfate at hydrophobic surfaces under physiological conditions, the effects of Mg2+ were the reverse and of a much smaller magnitude. Similar findings were obtained for proteodermatan/chondroitin sulfate, as well as for the heparan sulfate side chains of proteoheparan sulfate. Furthermore, both Na+ and K+ cause an increase in the adsorption in certain concentration ranges, due to electrostatic reasons. However, Na+ and K+ were also found to oppose the effects of Ca2+. Moreover, the effects of K+ are of a smaller magnitude than those of Na+ and occur over a longer time scale. Thus, general electrostatic effects, as well as cation-specific ion-binding effects, are of importance for the biological performance of proteoglycans, e.g., in the endothelial cell membrane.