Structural characteristics of the palmitoyl and dodecyl chains as well as of the headgroups of POPC and C12E4, respectively, were determined at 25 degrees C in the pure and mixed (surfactant lipid molar ratios R-S/L. = 0.2 and 1) membranes hydrated at the same relative humidity of 97% by deuterium NMR using partially deuterated lipids (POPC-d(31) and POPC-d(4)) and surfactants (C12E4-d(25) and C12E4-d(16)) At R-S/L = 0.2 the surfactant causes a rigidization of the membrane evidenced by the reduction of the cross-sections of the hydrocarbon chains, mainly of that of the surfactant dodecyl chain, whereas at R-S/L = 1 fluidization of the membrane occurs due to the mismatch of the hydrocarbon chain lengths. The surfactant dodecyl chain penetrates the membrane to the same depth as the first twelve methylene segments of the lipid palmitoyl chain. Experiment and Monte Carlo simulation show that the mean orientation of the lipid headgroup simultaneously changes toward the membrane director whereas the headgroup structure adopts a pearlike shape within the lipid matrix (R-S/L = 0.2),which changes its shape, increasing the surfactant concentration (R-S/L = 1) Conclusions on the prevailing interactions have been drawn. The successful analysis of the surfactant headgroup H-2 NMR spectra adds＇ a new example to the potentialities of the modified de-Pakeing techniques developed by Schafer et al.(22,23).