The impact of low cholesterol concentrations on an egg sphingomyelin bilayer is investigated using P-31 magic angle spinning (MAS) NMR spectroscopy. The magnitude of the isotropic P-31 MAS NMR line width is used to monitor the main gel to liquid crystalline phase transition, along with a unique gel phase pretransition. In addition, the P-31 chemical shift anisotropy (CSA) and spin-spin relaxation times (T-2), along with the effects of spinning speed, proton decoupling and magnetic field strength, are reported. The variation of this unique gel phase thermal pretransition with the inclusion of 5 through 21 mol% cholesterol is presented and discussed. (c) 2007 Elsevier B.V. All rights reserved.