From analyses of various samples of essential oils extracted by hydrodistillation from the branches ofThuja occidentalisL. andAbies Balsamea(L.) Mill., it was shown that there are extensive variations in the concentrations of the main components. The content in thujones and fenchone reaches a maximum value of 80 % after 40–50 minutes for cedarleaf oil. After 60 minutes, the concentrations in α-pinene and β-pinene in balsam fir oil are maximum at 15% and 40%, respectively.