We have developed a new spectroscopic method (excimer method) for detecting the interdigitated gel phase (L beta I) in phospholipid vesicles. This method is based on the disappearance of an excimer fluorescence peak of pyrenephosphatidylcholine (pyrene-PC) in the L beta I phase. Using this method we have studied the phase transition from gel phase (L beta') to L beta I phase of dipalmitoylphosphatidylcholine multilamellar vesicles in the presence of ethanol or ethylene glycol (EG). In both the cases of ethanol and EG, a sharp decrease in the ratio of excimer to monomer fluorescence intensity (E/M) of pyrene PC appeared at the same concentration of the transition from L beta' to L beta I as determined by the X-ray diffraction method or the scanning density method. After the transition to the L beta I phase, E/M values became very low. This excimer method enables us to detect the L beta I phase in unilamellar vesicles of phospholipids, which can hardly be studied by other methods such as X-ray diffraction.