The phase behavior of pseudobinary mixtures DMPC/C12E4 at high water excess was studied between 0 and 50 degrees C as a function of the surfactant mole fraction x using DSC and FTIR. The phase transitions of each component could be followed separately by FTIR spectroscopy mixing acyl-chain-deuterated DMPC-d(54) and alkyl-chain-protonated C12E4. Changes of the corresponding methylene stretching band frequencies yield the solidus and liquidus lines in the temperature-composition phase diagram. Two eutectics appear at x approximate to 0.55 and x approximate to 0.85. In the concentration range in between these values, the existence of a gel compound complex of about 1:2 lipid:surfactant composition was established. A peritectic point was suggested at low surfactant concentration between the rippled, gel, and liquid crystalline states of the lipid-rich bilayers on the basis of the Gibbs phase rule. A micellar transition (cloud point) precedes the formation of lamellar aggregates as the surfactant rich mixtures are heated. The phase transformations are discussed in terms of fluidity and hydration of the molecules on the basis of spectral data obtained from selected infrared absorption bands of the lipid and of the surfactant. The phase behavior of C12E4/DMPC was compared with literature data of related systems with respect to the chain melting transition, membrane solubilization, and complex formation.