The chemistry of 2-iodoacetic acid on Cu(100) has been studied by a combination of reflection-absorption infrared spectroscopy (RAIRS), X-ray photoelectron spectroscopy (XPS), temperature-programmed reaction/desorption (TPR/D), and theoretical calculations based on density In theory for the optimized intermediate structures. In the thermal decomposition of ICH2COOH on Cu(100) with a coverage less than a half monolayer, three surface intermediates, CH2COO, CH3COO, and CCOH, are generated and characterized spectroscopically. Based on their different thermal stabilities, the reaction pathways of ICH2COOH on Cu(100) at temperatures higher than 230 K ire established to be ICH2COOH -> CH2COO + H + I, CH2COO + H -> CH3COO, and CH3COO -> CCOH. Theoretical calculations suggest that the surface CH2COO has the skeletal plane, with delocalized pi electrons. approximately parallel to the surface. The calculated Mulliken charges agree with the detected binding energies for the two carbon atoms in CH2COO on Cu(100). The CCOH derived from CH3COO decomposition has a CC stretching frequency at 2025 cm(-1), reflecting its triple-bond character which is consistent with the calculated CCOH structure on Cu(100) Theoretically, CCOH at the bridge and hollow sites has a similar stability and is adsorbed with the molecular axis approximately perpendicular to the surface. The TPR/D study has shown the evolution of the products of H-2, CH4, H2O, CO, CO2, CH2CO, and CH3COOH from CH3COO decomposition between 500 and 600 K and the formation of H-2 and CO from CCOH between 600 and 700 K. However, at a coverage near one monolayer, the major species formed at 230 and 320 K are proposed to be ICH2COO and CH3COO. CH3COO becomes the only species present on the surface at 400 K. That is, there are two reaction pathways of ICH2COOH -> ICH2COO + H and ICH2COO + H -> CH3COO + I (possibly via CH2COO), which are different from those observed at lower coverages. Because the C-I bond dissociation of iodoethane on copper single crystal surfaces occurs at similar to 120 K and that the deprotonation of CH3COOH on Cu(100) occurs at similar to 220 K, the preferential COOH dehydrogenation of monolayer ICH2COOH is an interesting result, possibly due to electronic and/or steric effects.