The influence of electrostatic interactions on protein adsorption was studied by comparing the adsorption of lysozyme and succinylated lysozyme at silica surfaces. The succinylation affects the charge of the protein, but also the stability. Although changes in stability can have an influence on adsorption, our data show that the primary effect can be entirely understood in terms of electrostatic interactions. The adsorbed amount as a function of pH has a maximum for both proteins. This maximum coincides with the isoelectric point for succinylated lysozyme, and is close to the isoelectric point for lysozyme. At pH values where the protein is electrostatically repelled by the sorbent, higher ionic strengths increase adsorption, and for electrostatic attraction higher ionic strengths decrease adsorption. © 2004 Elsevier B.V. All rights reserved. Chemicals / CAS lysozyme, 9001-63-2; silicon dioxide, 10279-57-9, 14464-46-1, 14808-60-7, 15468-32-3, 60676-86-0, 7631-86-9; Muramidase, EC 3.2.1.17; Proteins; Silicon Dioxide, 7631-86-9; Succinates.