The chemisorption of glycine and its fully deuterated analogue, d 5 -glycine, deposited on the clean and Na modified Si(100)-2x1 surfaces, has been studied by high-resolution electron energy loss spectroscopy (HREELS) and ab initio methods. HREELS indicates that chemisorption of glycine and d 5 -glycine on Na/Si(100)-2x1 occurs through cleavage of the OH and OD bonds in the carboxylic groups. The adsorbed species are bonded through the formation of an ionic bond between the deprotonated carboxylate group and the adsorbed Na atom, leading to species in a bidentate coordination. In contrast, we observe monodentate glycinate and d 5 -glycinate species following the exposure of Si(100)-2x1 to glycine and deuterated glycine. The thermal stability of glycine on clean Si(100)-2x1 and on the Na modified surface has been studied by heating the substrate to various temperatures.