Poly(ethylene glycol)-induced fusion of two different vesicle systems has been examined: dipalmitoylphosphatidylcholine (DPPC) large unilamellar vesicles (LUV) and cardiolipin (CL)/dioleoylphosphatidylcholine (DOPC) (1:10) LUVs. A slight perturbation was established in the outer leaflets of DPPC LUVs by hydrolyzing 0.8% of the outer leaflet lipid with phospholipase A2 to produce lysophosphatidylcholine and palmitate which were then removed by bovine serum albumin. Similarly, 5 mM Ca2+ was added to the external compartment of CL/DOPC LUVs to alter the shape of the CL molecule and thereby create a perturbation in the outer leaflet packing of these vesicles. Contents mixing assays showed that both vesicle systems fused only when the outer leaflets of both contacting vesicles were perturbed as described. Two fluorescent probes (C6-NBD-PC and TMA-DPH) were used to detect changes in outer leaflet molecular packing between nonfusing and fusing systems. The steady-state fluorescence intensity of C6-NBD-PC added externally to either fusing system was enhanced relative to that of nonfusing vesicles. Phase-resolved measurements of probe lifetime showed that this was due mainly to enhanced partitioning of probe from a micellar state into fusing versus nonfusing membranes. Similarly, TMA-DPH was found to undergo more rapid motion when incorporated into fusing as opposed to nonfusing vesicles. The effects of deuterium exchange on probe lifetime also indicated that C6-NBD-PC and TMA-DPH penetrated more deeply into fusing than into nonfusing membranes. These results suggest that the fusogenic perturbations produced in these two very different lipid systems took the form of altered outer leaflet packing. We conclude that, for the two model lipid bilayers examined, small perturbations in lipid packing within contacting bilayer leaflets are necessary and probably sufficient to promote membrane fusion.