Phellandrene is a pair of organic compounds that have an analogous molecular structure and like chemical properties. α-Phellandrene and β-phellandrene are cyclical monoterpenes and are bonded isomers. In α-phellandrene, both double bonds are endocyclic and in β-phellandrene, one of them is exocyclic. Both are insoluble in water but miscible with ether.

α-Phellandrene is named after Eucalyptus phellandrene, now called Eucalyptus Radiata, from which it can be secluded. It is also a constituent of the essential oil of Eucalyptus dives. β-Phellandrene has been secluded from the oil of water fennel and Canada balsam oil.

The phellandrene in Eucalyptus oils has typically a high rotation to the left so that the normal optical rotations of the crude oils of some species is perceptible to the influence of this terpene, subsequently, the rotation constant is less steady with the phellandrene having Eucalyptus oils than with those in which the prime terpene is pinene. This difference is more particularly marked with the oils of those classes in which dextro-rotatory pinene and lavo-rotatory phellandrene ensue together in some quantity, and the volatility of rotations of the phellandrene in these cases is noticeable. The effect of dextro-phellandrene is also shown in some of the more definite phellandrene oils at certain times of the year.

Unlike pinene, which ensues in both active forms, phellandrene in any Eucalyptus oil has not yet been found in which the dextro-rotatory modification prevails, although the great differences shown in the scale of rotations with the oils of the numerous species in which this terpene occurs, suggests the occurrence of both active forms. Lesvos-phellandrene is thus the major form in Eucalyptus oils, and as essentially all those of the phellandrene-bearing Eucalypts have now been examined, it is hardly likely that the dextrorotatory alteration will now be found to be present in excess in the oils of this type.

Phellandrene in Eucalyptus oils is often linked with lava-rotatory piperidone, but not always, though it may be accepted that the most definite phellandrene Eucalyptus oils always comprise of this ketone in smaller or larger amounts, in normal distillates from 5 per cent. in that of E. radiate, to 40 per cent. in that of E. dives.
It is redundant to enumerate here those Eucalyptus types in the oils of which phellandrene has been spotted, as they are somewhat abundant, but they will be found documented in the table of general characters and principal elements.

The Nitro site from the lava-rotatory phellandrene is strongly dextrorotatory, while that from dextro-phellandrene is lava-rotatory, but both are similar in the presence and melt at the same temperature. They have been shown by Wallach and Bieschke (loc. cit.) to form two isomeric nitro sites, which may be separated by crystallizing from acetone and alcohol; the lava-phal-and renew-a-nitro site melting at 112-113° C, and the lava-a-phellandrene-β-nitro site at 1050 C.; the dextrose forms also melted at the same temperature.

This phellandrene can also be used in fragrances because of their pleasing aromas. The odor of β-phellandrene has been described as peppery-minty and somewhat citrusy.
The α-phellandrene isomer can be hazardous and explosive peroxides on contact with air at elevated temperatures