[3-3H]Squalene and [3-3H]-2,3-oxidosqualene, key compounds for studying the biosynthesis of sterols, were synthesized. The main step was a modified Wittig reaction of [1-3H]trisnorsqualene aldehyde with a phosphorus ylide to give [3-3H]squalene or with a sulfur ylide to give [3-3H]-2,3-oxidosqualene. These compounds were obtained in high radiochemical yield and high purity under simple conditions.