The fusion of enveloped viruses to target membranes is promoted by certain viral fusion proteins. However, many other proteins and peptides stabilize bilayer membranes and inhibit membrane fusion. We have evaluated some characteristics of the interaction of peptides that are models of segments of measles and influenza fusion proteins with membranes. Our results indicate that these models of the fusogenic domains of viral fusion proteins promote conversion of model membrane bilayers to nonbilayer phases. This is opposite to the effects of peptides and proteins that inhibit viral fusion. A peptide model for the fusion segment of the HA protein of influenza increased membrane leakage as well as promoted the formation of nonbilayer phases upon acidification from pH 7-5. We analyze the gross conformational features of the peptides, and speculate on how these conformational features relate to the structures of the intact proteins and to their role in promoting membrane fusion.