The role of the oxidation state of cerium cations in a thin oxide film in the adsorption, geometry, and thermal stability of glycine molecules was studied. The experimental study was performed for a submonolayer molecular coverage deposited in vacuum on CeO2(111)/Cu(111) and Ce2O3(111)/Cu(111) films by photoelectron and soft X-ray absorption spectroscopies and supported by ab initio calculations for prediction of the adsorbate geometries, C 1s and N 1s core binding energies of glycine, and some possible products of the thermal decomposition. The molecules adsorbed on the oxide surfaces at 25 °C in the anionic form via the carboxylate oxygen atoms bound to cerium cations. A third bonding point through the amino group was observed for the glycine adlayers on CeO2. In the course of stepwise annealing of the molecular adlayers on CeO2 and Ce2O3, the surface chemistry and decomposition products were analyzed and found to relate to different reactivities of glycinate on Ce4+ and Ce3+ cations, observed as two dissociation channels via C-N and C-C bond scission, respectively. The oxidation state of cerium cations in the oxide was shown to be an important factor, which defines the properties, electronic structure, and thermal stability of the molecular adlayer.