Interaction of polyethylene glycols (PEGs) with phospholipid membranes leads to aggregation and fusion of membranes. The structural basis of these events in membranes, especially in contact with low-molecular-weight PEGs, is uncertain. Using phsopholipases, a class of interfacially active enzymes, we demonstrate enhanced accessibility of lipid hydrophobic portions in the presence of PEGs. All three phospholipases, i.e., A(2), C, and D, show enhancement of activity in the presence of PEGs. Enhancement of activity does not depend on the size of the vesicle or the presence of proteins in the membrane. Fluorescence quenching of probes buried in the membrane supports the phospholipase data. The utility of phospholipases as probes to monitor local and fine structural changes in the membranes is discussed,