Abstract— The molecule (1,1'‐dipyrenyl)‐methyl ether [dipyme] shows intramolecular excimer fluorescence in competition with fluorescence from the locally excited pyrene chromophore. This intensity ratio Ic/Im is sensitive to solvent viscosity. The molecule is soluble in synthetic phospholipid membranes. 1H NMR studies suggest that the molecule is localized in the hydrocarbon region of the membrane. Fluorescence measurements at various temperatures of dipyme dissolved in these membranes show that Ic/Im is exceedingly sensitive to fluidity changes accompanying both the pretransitions and the gel to liquid crystalline transitions of the membrane. These studies can be carried out at a mole ratio of probe to lipid 102‐103 smaller than that necessary to observe bimolecular pyrene excimer formation.