Intrinsic structural defects in the lamellar (Lα) phase of the system sodium dodecyl sulphate (SDS)/decanol/water are studied by a combination of NMR2H quadrupole splittings of α-deuteriatedSDSand small angle X-ray scattering (SAXS). The focus is on the variation of the density and size of the defects with the bilayer composition (decanol/SDSmol ratio 0.46–2.5), the bilayer volume fraction (0.27–0.64), and the temperature (20–40°C). It is found that bilayer defects are promoted by a small decanol/SDSratio in the bilayers and by a low bilayer volume fraction, i.e. the same factors that drive the progression of phases towards microstructures with more highly curved interfaces. Variations of the extent of defects in the Lαphase reflect variations of the number of defects rather than changes of their size. The data are consistent with either slit defects (slit width ∼ 10 Å) or pore defects (diameter ∼ 25 Å). In either case, the interface separation across the defect is considerably smaller than the inter-bilayer separation. There is no evidence of transitions between Lαphases with different defect patterns. The defect variation with composition is analysed in terms of a simple thermodynamic model, showing that, apart from the electrostatic bilayer repulsion (which opposes defects), it is necessary to allow for considerable variations of the defect self-energy (which promotes defects).