The influence of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), and 1,2-dilauroyl-sn-glycero-3-phosphocholin (DLPC) on the mixing behavior of phospholipids derived from 1,2-dimyristoyl-sn-glycero-3-phosphoethanolamine (DMPE) and 1,2-distearoyl-sn-glycero-3-phosphoethanolamine (DSPE) has been examined in the fluid bilayer state by means of nearest-neighbor recognition methods (Davidson, S. K. M.; Regen, S. L. Chem. Rev. 1997, 97, 1269). When disulfide-based dimers derived from DMPE and DSPE were allowed to undergo monomer exchange at 60 degrees C, in the presence of 50 mol % of DPPC, an equilibrium molar ratio of heterodimer to each homodimer was found to be 1.95 +/- 0.07; with DMPC as the diluent, a small but significant level of nearest-neighbor recognition was observed (i.e.; the dimer ratio was 1.86 +/- 0.02). In sharp contrast, the use of DLPC resulted in a very strong demixing effect, which was characterized by a dimer ratio of 0.88 +/- 0.04. The mechanistic and practical implications of these findings are briefly discussed.