Molecular dynamics simulations have been used to study the properties of mixed dioleoylphosphatidylcholine/ dioleoylphospatidylethanolamine bilayers as a function of phosphatidylcholine (PC)/phosphatidylethanolamine (PE) head-roup composition. The equilibrium properties of mixed PC/PE bilayers were found to be nonlinear in the composition. The properties studied show saturation behavior at a PC/PE ratio of 1:3. The effect of adding small amounts of PE to PC bilayers is a strong reduction of the area of the bilayer. The effect of adding small amounts of PC to PE bilayers is an increased hydration of the PC headgroups compared to PC hydration in pure PC bilayers. Nearest-neighbor analysis did not indicate lateral ordering of the lipids for any mixture. The observed trends are discussed in terms of the size of, and the interactions between, PC and PE headgroups.