We have obtained force vs. separation relations between bilayers in 10 different phospholipid preparations: dilauroyl-dimyristoyl-, dipalmitoyl-, distearoyl-, or dioleoylphosphatidylcholine (PC); egg phosphatidylethanolamine; cholesterol-containing bilayers of dipalmitoyl PC and of egg PC. The chemical potential of water in the multilamellar lattice is determined at all water contents and changes continuously with bilayer separation; no discrete classes of water are observed. The interbilayer van der Waals force is estimated from the balance of forces at the bilayer separation where the multilayer lattice is in equilibrium with pure water. Although quantitative differences are evident for different phospholipids, all force curves but one show a clear, exponentially decaying "hydration "repulsion" whose decay distance is 2-3 A . Estimates of forces between bilayer vesicles show great sensitivity to the identity of the phospholipid polar group and to the packing of the hydrocarbon acyl chains. On major implication of this is the likelihood of local structural changes and lipid segregation in the area of closest approach of interacting vesicles of mixed phospholipids.