The plant sterols sitosterol and stigmasterol exert very different effects on plant model membranes, the first one being a "reinforcer" like cholesterol, the second one not. 25-(2)H-Stigmasterol has been synthesized by coupling of the 22-aldehyde derived from stigmasterol by ozonolysis, with the proper sulfone labeled in position 25. The configuration of the ethyl side chain at C-24 was controlled by separation of the diastereomers introduced via a chiral sulfoxide. This synthetic scheme allowed the introduction of a labeled side chain in plant sterols in eight steps for stigmasterol and nine for sitosterol (overall yield ca. 15%). Using both diastereomers, the 24-epimers of sitosterol (clionasterol) and stigmasterol (poriferasterol) have also been synthesized. Deuterium NMR on oriented lipid bilayers made of soybean phosphatidylcholine and containing these four labeled plant sterols clearly reveals the difference of orientation and mobility of the four side chains.