Low-angle X-ray diffraction is a powerful method to analyze the structure of membrane bilayers. Specifically, the technique can be used to determine accurately the thickness of fully hydrated bilayers. Herein details are presented showing how this technique can measure the difference in thickness of bilayers in detergent-resistant membranes and detergent-soluble membranes extracted from model systems known to contain both raft and nonraft domains. The observed thickness difference may be critical in the sorting of transmembrane proteins between raft and nonraft bilayers.