Deuterium NMR is used to explore the phase behavior of membranes containing phospholipids and ergosterol. The temperature-ergosterol mole fraction phase diagram constructed from the NMR data will be discussed. Our approach to analyzing the NMR spectra has allowed us to definitively locate and characterize the region where two liquid crystalline phases coexist. The general methodology for phase boundary determination in phospholipid/sterol membranes is presented.