The transmembrane domains of fusion proteins areknownto beimportant for their fusogenic activity. Inaneffort to systematically investigate the structure/function relationships of transmembrane domains we had previously designed LV-peptides that mimic natural fusion proteinTMDsin their ability to drive fusion after incorporation into liposomalmembranes. Here,weinvestigate the impact of different structural features of LV-peptide TMDs on inner and outer leaflet mixing. We find that fusion driven by the helical peptides involves a hemifusion intermediate as previously seen for natural fusion proteins. Helix backbone dynamics enhances fusion by selectively promoting outer leaflet mixing. Furthermore, the hydrophobic length of the peptides as well as covalent attachment of long acyl chains affects outer and inner leaflet mixing to different extents. Different structural features of transmembrane domains thus appear to differentially influence the rearrangements of lipids in fusion initiation and the hemifusion-to-fusion transition. The relevance of these findings in respect to the function of natural fusion proteins is discussed.