We used fluorescent resonance energy transfer (FRET) measurements of large unilamellar vesicles to investigate domain formation in lipid mixtures composed of 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC) and equimolar portions of cholesterol (Chol) and an ordered, high-melting phospholipid: N-palmitoyl-sphingomyelin (SM), porcine brain sphingomyelin (BSM) or 1,2-dipalmitoyl-phosphatidylcholine (DPPC). Mixtures with different concentrations of POPC were examined as a function of temperature. The FRET results are interpreted to indicate that at 25°C ordered and disordered lipid domains exist. As the temperature is increased to 40°C the area of domains of liquid ordered phase appears to decrease. When the temperature increases to 60°C the liquid disordered phase is the primary phase of the entire vesicle. All three mixtures show the same trends in domain size between the liquid disordered and liquid ordered phases. These FRET measurements are used for comparison when performing Monte Carlo simulations of these mixtures. This work has been supported by NSF grant CHE-1464769.