The temperature dependence of the Raman spectrum has been studied for binary phospholipid mixtures of dimyristoyl phosphatidylcholine (and its chain deuterated -d54 derivative) with distearoyl phosphatidylcholine. Two distinct melting regions are observed for the 1 : 1 mole ratio mixture. The use of deuterated phospholipid permits the identification of the lower (approximately 22 degrees C) transition with primarily the melting of the shorter chain component, and the higher (approximately 47 degrees C) transition primarily with the melting of the longer chains. The C-H stretching vibrations of the distearoyl component respond to the melting of the dimyristoyl component, an apparent consequence of alterations in the lateral interactions of the distearoyl chains. These changes in the C-H spectral region suggest that phase separation does not occur in the gel state for this system. The results are in reasonable accord with recent calorimetric studies (Mabrey, S. and Sturtevant, J.M. (1976) Proc. Natl. Acad. Sci. U.S. 73, 3862-3866). The feasibility of using deuterated phospholipids to monitor the conformation of each component in a binary phospholipid mixture is demonstrated.