A comparative study is conducted on the influence of two types of polymeric compounds on the phase behavior of 1.2-dihexadecanoyl-s,n-glycero- 3-phosphotidylcholine (DC₁₆PC) lipid bilayers. The first polymeric compound is a lipopolymer, with two different lengths of a hydrophilic polyethylene oxide moity, anchored to the bilayer by a 1,2-dioctadecanoyls,n-glycero-3- phosphoethanolamine (DC₁₈PE) lipid. The second type, which is a novel type of membrane-spanning object, is an amphiphilic tri-block copolymer composed of two hydrophilic stretches of polyethylene oxide separated by a hydrophobic stretch of polystyrene. Hence the tri-block copolymer may act as a membrane- scanning macromolecule mimicking an amphiphilic protein or polypeptide. Differential scanning calorimetry is used to determine a partial phase diagram for the lipopolymer systems and to assess the amount of lipopolymer that can be loaded into DC₁₆PC lipid bilayers before micellization takes place. Unilamellar and micellar phase structures are investigated by fluorescence quenching using bilayer permeating dithionite. The chain length- dependent critical lipopolymer concentration, denoting the lamellar-to- micellar phase transition, compares favorably with a theoretical prediction based on free-energy considerations involving bilayer cohesion and lateral pressure exerted by the polymer chains.