Western red cedar leaf affords an essential oil characterized by high thujone content. Students in an advanced organic chemistry lab course isolate a single thujone diastereoisomer from commercially available cedar leaf oil. Treatment of crude oil, containing roughly 70% thujone, predominately as [alpha]-thujone (6.5:1), with ethanolic sodium hydroxide leads to epimerization at the chiral [alpha]-carbon. Equilibration thus inverts the diastereoisomeric ratio in favor (roughly 2:1) of the [beta]-thujone epimer. Selective formation of the bisulfite addition product from the [beta]-isomer allows for a simple purification via filtration. Release of the free ketone upon treatment with Na[subscript 2]CO[subscript 3] yields highly pure [beta]-thujone, which is characterized and identified using IR, MS, and NMR spectroscopy. (Contains 1 figure and 1 scheme.)