The mechanism and kinetics of aspartic acid (Asp, HO2CCH(NH2)CH2CO2H) decomposition on Cu(100) have been studied using X-ray photoemission spectroscopy and temperature programmed reaction spectroscopy. We investigate the Asp decomposition mechanism in detail using unlabeled D-Asp and isotopically labeled L-Asp-4-C-13 (HO2CCH(NH2)CH2 (CO2H)-C-13), L-Asp-d(7) (DO2CCD(ND2)CD2CO2D), L-Asp-2,3,3-d(3) (HO2CCD(NH2)CD2CO2H), and L-Asp-N-15-2,3,3-d(3) (HO2CCD((NH2)-N-15)CD2CO2H). The monolayer of Asp adsorbed on the Cu(100) surface is in a doubly deprotonated bi-aspartate form (-O2CCH(NH2)CH2CO2-). During heating, Asp decomposes on Cu(100) with kinetics consistent with a vacancy-mediated explosion mechanism. The mechanistic steps yield CO2 by sequential cleavage of the C3-C4 and C1-C2 bonds, and N CCH3 and H-2 via decomposition of the remaining CH(NH2)CH2 intermediate. Deuterium labeling has been used to demonstrate that scrambling of H(D) occurs during the decomposition to acetonitrile of the CD(NH2)CD2 intermediate formed by decarboxylation of L-Asp-2,3,3-d(3) and L-Asp-N-15-2,3,3-d(3).