Peptide-induced vesicle fusion is frequently accompanied by leakage of vesicle contents. To determine the correlation between these two processes, we studied the interaction of the amphiphilic peptide pardaxin and two of its analogues with large unilamellar vesicles composed of phosphatidylserine. A pardaxin analogue with a positive charge at both its C- and N-termini induced significantly more fusion but less leakage than the parent peptide. Fusion and leakage were studied with large unilamellar vesicles of two sizes. Aggregation of vesicles was found to be the rate-limiting step in the overall fusion process induced by the peptides. The rates and extents of fusion, determined by membrane mixing, increase in vesicle size, and mixing of aqueous contents, were significantly enhanced in the presence of 2.5-5 mM Mg2+ which promoted vesicle aggregation. Model calculations showed that increasing the peptide to lipid ratio resulted in a parallel increase in the fusion rate constants. As the average vesicle diameter was increased, the extent of leakage was enhanced, as more peptide molecules were bound to each vesicle. The mode of leakage induced by the peptides was also investigated. Our results suggest that the potency of a peptide to induce vesicle fusion is not necessarily associated with its capacity to induce leakage, and we further elucidate how these capacities depend on the structures of the peptides.