The hydroxycinnamic acids (HCAs) in foods have been suggested to contribute to the flavor properties by multiple mechanisms. In this paper we investigate the reactivity of hydroxycinnamic acids in simple aqueous Maillard model systems on flavor development. Using isotope labeling techniques, the hydroxycinnamic acids were reported to form adducts with transient Maillard reaction flavor precursors, hexose fragments, as well as with amino acids and/or amino acid reaction products. The generation of Maillard-type volatile compounds, such as pyrazines, cyclotene, diacetyl, 2-acetyl-l-pyrroline in the model systems, was also suppressed by addition of HCAs. In summary, the HCAs were reported to alter the mechanisms of Maillard flavor development in these aqueous model systems.