Aminoethylcysteine ketimine decarboxylated dimer (AECK-DD) is a natural compound with antioxidant properties of a new family of sulfur-containing amino acids. It has been detected in human urine and plasma, in mammalian cerebellum, and in dietary vegetables. In this study, we first demonstrate the absorption of AECK-DD in mice from AECK-DD-supplemented diet, using both liquid chromatography with electrochemical detection and gas chromatography coupled with mass spectrometry. AECK-DD circulates in the plasma of supplemented mice at a micromolar concentration and is incorporated in liver tissue. The absorption of AECK-DD is dose dependent. The dehydrogenation product of AECK-DD was also identified in plasma and liver of mice fed the AECK-DD-supplemented diet. A significant increase in plasma antioxidant potential was measured in mice fed AECK-DD-supplemented diet with respect to mice fed the control diet. These results demonstrate for the first time the absorption of AECK-DD from diet and the physiological relevance of this compound through its antioxidant action in vivo.