The mixing properties of exchangeable phospholipids, derived from 1,2-dipalmitoyl-sn-glycero3-phosphoethanolamine and 1,2-distearoyl-sn-glycero-3-phosphoethanolamine, with an exchangeable form of cholesterol have been used to monitor the transition from the liquid-disordered to the liquid-ordered phase in cholesterol-containing bilayers, made from 1,2-dipaimitoyl-sn-glycero-3-phosphocholine and 1,2-distearoyl-sn-glycero-3-phosphocholine, respectively.