Planar supported lipid bilayers (SLBs) are often studied as model cell membranes because they arc accessible to a variety of surface-analytic techniques. Specifically, recent studies of lipid phase coexistence in model systems suggest that membrane lateral organization is important to a range of cellular functions and diseases. We report the formation of phase-segregated dioleoylphosphatidylcholine (DOPC)/sphingomyelin/cholesterol bilayers on mercaptoundecanoic-acid-modified (111) gold by spontaneous fusion of mid:midi:if vesicles, without the use of charged or chemically modified headgroups. The liquid-ordered (1(o)) and liquid-disordered (1(d)) domains are observed by atomic force microscopy (AFM) height and phase imaging. Furthermore, the mechanical properties of the bilayer were characterized by force indentation maps. Fits of force indentation to Sneddon mechanics yields average apparent Young's moduli of the 1(o) and 1(d) phases of 100 +/- 2 and 59.8 +/- 0.9 MPa, respectively. The results were compared to the same lipid membrane system formed on mica with good agreement, though modulus values on mica appeared higher. Semiquantitative comparisons suggest that the mechanical properties of the 1(o) phase are dominated by intermolecular van der Weals forces, while those of the fluid Id phase, with relatively weak van der Waals forces, are influenced appreciably by differences in surface charge density between the two substrates, which manifests as a difference in apparent Poisson ratios.