Proton and deuterium order parameters measured for the liquid crystalline phase of unsonicated lipid bilayer membranes are interpreted in terms of two motions: (i) chain reorientation and (ii) chain isomerization via kink diffusion. The observed order parameters are found to be compatible with angular deflections of the chain of about 50 degrees with respect to the bilayer normal, coupled with a probability of trans orientation of a methylene segment in the upper part of the chain of about 0.8-0.9. The motional model can be shown to account for the dynamic properties of the membrane system as measured by nuclear magnetic relaxation measurements, assuming that the chain isomerization occurs at a rate of approximately 10(10) s-1 and chain reorientation at a rate of approximately 10(7) s-1. Analysis of proton and deuterium line-width data in terms of this model shows that sonication has the effect of increasing the rate and amplitude of chain reorientation without substantially changing the isomerization motion along the acyl chain. These conclusions are briefly compared with similar observations recently reported in Raman spectroscopic studies.