The formation of amyloid fibrils by the human islet amyloid polypeptide is associated with type II diabetes. While it was previously suggested that the formed fibrils are toxic to pancreatic β-cells due to membrane permeation activity, more recent studies suggested that protofibrillar assemblies have significantly higher potency in permeating lipid bilayers. Here, we specifically studied the membrane interaction activity of soluble and insoluble islet amyloid polypeptide assemblies at high temporal resolution. A colorimetric analysis using lipid/polydiacetylene (PDA) biomimetic vesicles clearly demonstrated the transient formation of soluble assemblies that strongly interact with the lipid vesicles. A peak in the level of membrane binding of the soluble fraction, as reflected by the colorimetric assay, was observed after incubation for ∼1 h, followed by a decrease in the level of membrane interaction of the assemblies. The transient nature of the membrane-active assemblies was independently confirmed by a fluorescence quenching assay. Ultrastructural analysis using transmission electron microscopy provided morphological evidence of prefibrillar assemblies, supported the transient existence of membrane interacting soluble species, and facilitated observation of the non-membrane-active filaments in the solution. Taken together, our results provide experimental evidence for the formation of transient soluble prefibrillar assemblies which are highly membrane-active. The implications of these observations are discussed in light of designed fibrillization inhibitors.