The retention and binding mechanisms in electrostatic interaction‐based chromatography (ion‐exchange chromatography) of PEGylated proteins (covalent attachment of polyethylene glycol chains to protein) were investigated using our previously developed model. Lysozyme and bovine serum albumin were chosen as model proteins. The retention volume of PEGylated proteins shifted to lower elution volumes with increasing PEG molecular weight compared with the non‐modified (native) protein retention volume. However, PEGylation did not affect the number of binding sites appreciably. The enzyme activity of PEGylated lysozyme measured with a standard insoluble substrate in suspension decreased considerably, whereas the activity with a soluble small‐molecule substrate did not drop significantly. These findings indicate that when a protein is mono‐PEG‐ylated, the binding site is not affected and the elution volume reduces due to the steric hindrance between PEGylated protein and ion‐exchange ligand.