Using a novel, FRET-based strategy, we have been able to make feasible very high-resolution studies of phase behavior in so-called “lipid raft” membrane mixtures. These studies are carried out on ordinary, polydisperse vesicle suspensions, so we have also been able to prepare our samples according to a procedure designed specifically to guard against artifactual phase separation. In some respects (i.e., the number and nature of two-phase regions observed), our phase diagrams are consistent with previously published reports. However, in other respects (i.e., overall size of miscibility gaps, phase boundary locations and their dependence on temperature) there are clear differences. Comparisons between our results and previously reported phase boundaries suggest that lipid-raft mixtures may be particularly susceptible to demixing effects during sample preparation. In addition, our high-res studies of three closely related mixtures (DOPC/DPPC/Chol, DLPC/DPPC/Chol, POPC/DPPC/Chol) reveal dramatic similarities and differences in phase behavior that shed light on the details of PC-Chol interactions.