The phase equilibria in mixtures of egg lecithin and cetyltrimethylammonium chloride in brine (100 mM sodium chloride) were studied with particular emphasis on the behavior of the lamellar phase. The solvent corner of this pseudoternary system features an extensive lamellar phase which we have characterized by means of cryo-transmission electron microscopy (cryo-TEM), small-angle X-ray diffraction, and H-2 and P-31 NMR. The cryo-TEM micrographs illustrate a smooth transition in aggregate microstructure between lamellar and micellar structures with a perforated lamellar structure as an intermediate state. The findings from X-ray diffraction and NMR spectroscopy also indicate a deviation from ordinary bilayer structures in the solvent rich region of the lamellar phase. This is concluded from strongly nonideal swelling and progressively decreasing H-2 NMR quadrupole splittings of the deuterated cetyltrimethyl-ammonium chloride inside the lamellar region. The correlation between the observed aggregate structures and the phase behavior is discussed in terms of a continuous transition from the lamellar to the micellar phase.