<p>The Maillard reaction of D-glucuronic acid (GlcA) with alanine was studied using ESI-qTOF-MS/MS, LC-ESI-qTOF-MS, Py-GC/MS, FTIR, isotope labeling and ¹³C-NMR to elucidate its reported unusual reactivity in the Maillard reaction. The results have indicated that this enhanced reactivity is linked to the specific stereochemistry of GlcA, which facilitates lactonization and formation D-glucuronolactone (GlcLA). The lactone is capable of assuming an <i>anti-coplanar</i> conformation, which in turn facilitates β-elimination, leading to enhanced formation of reactive intermediates such as 3-deoxy-osones and 1-amino-3-deoxy-4,5-dihydroxypentan-2-one (ADDP). GlcA exhibited more intense browning than D-galacturonic acid (GalA), which can be attributed to its ability to undergo lactonization. Both GlcA-alanine and GlcLA-alanine model systems could generate 3-deoxy-osones, however following different formation pathways. In both systems, the 3-deoxy-osone undergoes decarboxylation and Strecker degradation to yield ADDP. In the model system of GlcA, alanine primarily reacted with GlcA to form the corresponding Amadori rearrangement product, whereas in the lactone system, the generated amino compound ADDP reacted with the lactone present in the model system, to form an Amadori-type compound. These findings provide mechanistic insight into the enhanced Maillard reactivity of GlcA and highlight the critical role of lactonization in directing its browning pathways.</p>