A study of bilayer phase behavior is presented for the phospholipid mixture di12:0-PC/di16:0-PC, based on an analysis of equilibrium surface pressures which develop at the gas-water interface above suspensions of bilayer vesicles. At 22 +/- 1 degrees C, the fluidus and solidus bilayer phase boundaries are determined to be 32 and 85 mol % di16:0-PC, respectively, consistent with literature values obtained through calorimetry. Evidence is presented which suggests that Teflon substrates are better suited than high-energy substrates for surface pressure measurements involving insoluble monolayers.