Biological membranes can perform a variety of different functions, and their variable compositions are a reflection of this fact.  Recently, there has been a lot of interest in the possible roles that may be played by localized biomembrane environments (a.k.a. membrane domains, or “rafts”).  As part of a larger effort to understand membrane domains, our laboratory at      Colgate is developing techniques for mapping out the conditions under which alternative membrane environments will form within a mixture of membrane components.  Using Fluorescence Resonance Energy Transfer between differentially partitioning membrane probe molecules, we can identify the membrane compositions that spontaneously separate into     distinct coexisting environments.  As a demonstration of the potential of this technique, we will present results from a temperature-dependent study of a 2-component membrane mixture.