- PEGylation of therapeutic proteins can enhance their efficacy as biopharmaceuticals through increased stability and hydrophilicity, and decreased immunogenicity. A site-specific PEGylated protein (e.g. mono-PEGylated at N-terminus) is frequently desirable as a product. However, multiple-PEGylated forms are frequently produced as byproducts. In this paper we discuss the fractionation of the different PEGylated forms of a protein by hydrophobic interaction chromatography using a stack of hydrophilized PVDF membrane, which has been shown to be environment responsive, as stationary phase. With the model protein examined in this study (i.e. lysozyme), the apparent hydrophobicity in the presence of a lyotropic salt increased with the degree of PEGylation. Based on this, unmodified lysozyme and its mono-, di- and tri-PEGylated forms could each be resolved into separate chromatographic peaks. Such fractionation was not feasible using conventional hydrophobic interaction chromatography using a butyl column. The use of membrane chromatography also ensured that the fractionation was fast and hence suitable for analytical applications such as product purity determination and monitoring of the extent of PEGylation reactions.