Photon correlation spectroscopy has been used to determine the degree and sizes of PEGylated-liposome aggregation. The aggregation of the liposomes has been induced by NeutrAvidin-biotin adsorbed to the surfaces of the vesicles. None of the liposome preparations described herein exhibited aggregation before the introduction of NeutrAvidin. The critical ratios of the NeutrAvidin-biotin required to induce vesicle aggregation have been measured and are reported in this work. The aggregation is found to be dependent upon the ratio of NeutrAvidin to exposed biotinylated lipid and also the exposed biotinylated lipid fraction.