Cell membranes have a nonhomogenous lateral organization. Most information about such nonhomogenous mixing has been obtained from model membrane studies where defined lipid mixtures have been characterized. Various experimental approaches have been used to determine binary and ternary phase diagrams for systems under equilibrium conditions. Such phase diagrams are the most useful tools for understanding the lateral organization in cellular membranes. Here we have used the fluorescence properties of trans-parinaric acid (tPA) for phase diagram determination. The fluorescence intensity, anisotropy, and fluorescence lifetimes of tPA were measured in bilayers composed of one to three lipid components. All of these parameters could be used to determine the presence of liquid-ordered and gel phases in the samples. However, the clearest information about the phase state of the lipid bilayers was obtained from the fluorescence lifetimes of tPA. This is due to the fact that an intermediate-length lifetime was found in samples that contain a liquid-ordered phase and a long lifetime was found in samples that contained a gel phase, whereas tPA in the liquid-disordered phase has a markedly shorter fluorescence lifetime. On the basis of the measured fluorescence parameters, a phase diagram for the 1,2-dioleoyl-sn-glycero-3-phosphocholine/N-palmitoyl sphingomyelin/cholesterol system at 23 °C was prepared with a 5 mol % resolution. We conclude that tPA is a good fluorophore for probing the phase behavior of complex lipid mixtures, especially because multilamellar vesicles can be used. The determined phase diagram shows a clear resemblance to the microscopically determined phase diagram for the same system. However, there are also significant differences that likely are due to tPA's sensitivity to the presence of submicroscopic liquid-ordered and gel phase domains.