Delta 8 THC and Delta 9 THC are known as isomers, molecules with the same chemical formula but different structural relationships between their atoms. The difference in question is a double bond on the 8th carbon chain for delta 8, while it is on the 9th carbon chain for delta 9.

The difference lends to slightly altered physiological responses from these molecules. Generally, delta eight anecdotally yields a less potent psychoactive effect or “high” than delta 9.