By means of accurate spectral simulation from selected nitroxide spin probes, the Electron Spin Resonance (ESR) has proven an useful tool to monitor phase separations in lipid mixtures.  The agreement of ESR data with X-ray diffraction (XRD) data when the temperature is varied in order to induce phase separation supports the validity of the method.  The ESR spectral shape analysis can not only inform on the heterogeneity of the lipid systems but also identify a lamellar or hexagonal arrangement of lipids around the spin label as well as isotropic (liquid or cubic) domains with however some limitations related to the rotational and translational diffusion of the probe.  The technical simplicity of CW ESR and its high sensitivity as regard to XRD to analyze rare biological lipid extracts can be a clear-cut advantage.  An other advantage of the ESR is the determination of the fraction, the viscosity and the molecular order of each component of the lipid heterogeneous mixtures.

An outline of computational methods involved in spectral simulations is given, specifying heir domains of applicability according to the choice of the probe.  Examples of applications of these methods are taken from an ESR and XRD study on the phase changes induced by cholesterol addition to a mixture of hosphatidylethanolamine (PE), phosphatidylserine (PS) and sphingomyeline (SM).