Lamiaceae herbs, such as rosemary and sage, are known to have particularly strong antioxidant properties. This can be attributed to lipidsoluble abietane diterpenoid polyphenols, such as carnosol, as well as water-soluble rosmarinic acid. A lipidic rosemary extract, which contains carnosol and related diterpenes, has a high radical-scavenging antioxidant capacity, especially against lipid oxidation. Furthermore, it has been reported to have synergistic effects with other antioxidants, although the mechanism is not fully understood. Our recent research examines the interactions of carnosol and related diterpenoid polyphenols [carnosic acid (CA), carnosol (CAR), rosmanol (ROS), isorosmanol (isoROS), and epirosmanol (epiROS)] isolated from rosemary with other food components. The current paper reports the results of studies on the enhancement of the antioxidant function and its mechanism in the presence of a thiol compound as a model cysteine-containing food ingredient.