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Edward J. ShimshickJ and Harden M. McConnell* abstract: The phase diagrams of aqueous dispersions of binary mixtures of dimyristoyl-, dipalmitoyl-, and distearoylphosphatidylcholine and dipalmitoylphosphatidylethanol-amine have been determined. The method usedis based on the partition of the spin label, 2, 2, 6, 6-tetramethylpiperidine-1-oxyl (Tempo) between the fluid hydrophobic regions of the lipids and the aqueous regions. As the hydrophobic regions of the phospholipid mixtures “freeze,” Tempo is excluded and its solubility in the lipids decreases. This solubility, mea-sured by means of a Tempo spectral parameter for different n