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The influence of a cationic charged polyelectrolyte, poly(diallyldimethylammonium chloride) on the interlayer spacing of the lamellar liquid crystalline system Na-dodecylsulfate (SDS)/decanol/water was investigated by means of low-angle X-ray diffraction, polarized microscopy and rheology. It could be demonstrated that a lamellar liquid crystalline phase was still formed by introducing a cationic polymer of 5000 g/mol into the mesophase, whereas different phenomena are observed in dependence on the polymer concentration. The average thickness of the layer, that means the layer repeat unit, is not influenced in the range of lower polymer concentration (up to 1%). At 2% polymer a lamellar liquid crystalline phase formed by SDS and decanol molecules and a frozen structure including polymer chains existing side by side. In a range of 5% and 10% polymer the frozen structure is the dominant one. It is assumed that electrostatic interactions determine the location of the polymer in the water layer between the surfactant head groups. Rheological investigations support the proposed model.