Planar-supported phospholipid bilayers are increasingly used as synthetic membranes for scientific and practical applications. The thermotropic phase properties of supported bilayers are important for recreating biologically relevant situations. Unlike free-standing lipid membranes that undergo one gel-to-fluid or main phase transition, mica-supported single bilayers have been found to undergo two separate leaflet transitions. Although the distinctive nature of the main transition in mica-supported bilayers has been attributed to different effects, determining their relevance has been problematic because vesicle fusion, the technique most widely used to prepare solid-supported bilayer membranes, does not allow one to readily control the lipid surface coverage and molecular density. To circumvent the limitations of the vesicle fusion method and systematically investigate the effects on the individual leaflet transitions of the lipid phase state and molecular density before deposition on the substrate, mica-supported single bilayers of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) were prepared using the Langmuir-Blodgett technique. The gel-to-fluid transitions of the bilayer leaflets were tracked by controlled-temperature atomic force microscopy to determine the relative fractions of the gel and fluid phases as a function of temperature. The fraction of solid versus temperature data was fit to the van't Hoff equation to determine the leaflet melting temperatures and transition enthalpies. The phase state and molecular density of the Langmuir monolayer precursor at the transfer pressure of 35 mN m(-1) was found to have a greater effect on the main transition temperature and width of the distal (upper) leaflet than that of the proximal (lower) one. The contributions of substrate-mediated condensation, asymmetric lipid densities, and surface area available for thermal expansion of the bilayer are addressed. This work demonstrates the potential of the Langmuir-Blodgett technique as a tool for identifying and manipulating the factors that govern the phase transition properties of surface-confined lipid bilayers.