Supported lipid bilayers (SLBs) are synthetic ultrathin organic membranes which serve as model systems for cell membrane and are promising for future applications in diagnostic devices or for biomimetics. The pathway of SLB formation is yet partially understood. In the present study, the transformation of spherically closed lipid bilayers to supported lipid bilayers in aqueous media in contact with SiO2 surfaces was studied. The adsorption kinetics of small unilamellar vesicles composed of dimyristoyl-sn-glycero-3-phosphatidylcholine (DMPC) on SiO2 surfaces were investigated using dissipation enhanced quartz crystal microbalance (QCM-D) as a function of buffer composition, especially sodium chloride concentration. The lipid used here possesses a phase transition temperature (T-m) of 24 degrees C which is close to the ambient and thus considerably higher than most other systems studied by QCM-D. With HEPES or Tris(.)HCl solutions containing sodium chloride (150 MM) and/or calcium chloride (2 mM), intact vesicles adsorb on the surface until a critical density (Theta(c)) is reached. At close vesicle contact the transformation from vesicles to supported phospholipid bilayers (SPBs) occurs. This pathway of SPB formation is referred to as pathway 1. In absence of CaCl2, the kinetics of the SPB formation process are slowed down, but pathway 1 is still observed. In absence of sodium chloride, the passage through Theta(c) disappears and this behavior is referred to as pathway 2. The transition from pathway 1 to pathway 2 occurs at a sodium chloride concentration of about 75-90 mM. The role of sodium chloride in vesicle-substrate and vesicle-vesicle interactions is discussed. (c) 2005 Elsevier B.V. All rights reserved.