trans-3-Methyl-4-octanolide, also called trans-β-methyl-γ-octalactone is a chemical compound of the lactone family with formula C9H16O2.

The (3S,4R) form is found in whiskey and other alcoholic beverages that have been aged in oak barrels, together with the more important cis isomer.

[1][2] It has a coconut, celery or fresh wood aroma, that can be detected by humans at the concentration of 20 μg/L in air.

[3] A mixture of the cis and trans isomers is repellent for mosquitos and flies.

[4] The (3S,4R) isomer is extracted by the alcoholic beverage from some precursor substances in the oak wood.