Publication details, including instructions for authors and subscription information: ABSTRACT: Composition of the heartwood oil of Thuja occidentalis L. was investigated by a combination of capillary GC (retention indices) and GUMS. The oil was obtained by steam &stillation ofvarious lots ofwaste residues from cedar shingle industries. Twelve out of 20 observed compounds were identified. Monooxygenated sesquiterpene compounds are the most important components (80% of the total). Occidentalol is the main component: its relative concentration varies between 20% and 50%, dependng on the isolation parameters. This concentration decreased as the isolation proceeded, typically from 40-50% to 10-20% between 1 and 5 hours of &stillation. Three occidol isomers, occidenol, and a-, p-, and y-eudesmols were also observed in relative concentrations of 4% to 12%. The monoterpene family is represented by terpinen-4-01, and carvacrol. a-Thujaplicin, a tropolone derivative, is also among the products. The relative concentration of thujaplicin decreased from 5% to 0% between 1 and 5 hours of distillation.