We have examined the structure of the lamellar phase (L-alpha) that coexists with a micellar solution (L-1) for a commercial sodium alkyl benzene sulfonate (LAS) mixed with water. The surfactant is a mixture containing C-10-C-13 alkyl chains, having all positional isomers of the benzene sulfonate group present except the 1-isomer. Unusually for ionic surfactants, the difference in compositions between the coexisting L-1 and L-alpha phases is large (L-1 = similar to 20 wt % LAS; L-alpha = similar to 65 wt %). The main technique employed was X-ray diffraction, supplemented by optical microscopy and differential scanning calorimetry (DSC). At ambient temperatures, the lamellar phase gives a single diffraction pattern with the main reflection (d) at similar to 32.5 A, whatever the composition. However, above 40 degrees C, the diffraction peak becomes broader and moves to higher d values. At higher temperatures still, several distinct and different diffraction peaks are observed, differing in detail according to composition. The largest d values (similar to 42-4 A) are observed for the lowest LAS concentrations, while the largest number of separate reflections (five) occurs for samples with similar to 44-50% LAS, both at the highest temperatures. Although there are some differences in the data between heating and cooling cycles, the d values return to the original value at low temperature. There are no observable transitions in DSC, nor is there any heterogeneity in the lamellar phase observable by microscopy. The data clearly indicate that there is some lateral separation of the different LAS isomers within the bilayers, which results in the formation of local lamellar regions having different surfactant compositions. This lateral phase separation may arise from the presence of an (electrostatic) attractive interaction, which gives rise to an upper consolute loop within the lamellar phase region of a pure LAS isomer. Similar mechanisms may occur in biological membranes and could be responsible for the occurrence of membrane lipid patches.