From The Official Website

Mellow Corn is produced in accordance with a unique set of regulations that define Straight Corn Whiskey. It is distilled from a mash of at least 80 percent corn and aged for four years in a once-used Bourbon barrel. Compared to the deep amber color created by the newly charred oak barrels in the production of our other Bourbons and American Whiskeys, the result of the used barrel is a lighter, straw-colored spirit. Heaven Hill Distillery is one of the few remaining producers of this unique whiskey.