A series of Pd/Al2O3 catalysts were prepared by incipient wetness impregnation method. Palladium loading was varied in a range of 0.125–4.0 mass%. The catalytic performance of the samples was tested in model reaction of CO oxidation at oxygen excess. Catalysts were characterized by temperature-programmed reduction (TPR), electron paramagnetic resonance, and UV–visible spectroscopy. Thermal aging of chosen samples was performed at 1000 °C. The thermal aging behavior was studied using TPR, X-ray diffraction analysis, transmission electron microscopy, and X-ray photoelectron spectroscopy (XPS). It was shown that palladium stabilized in the form of dispersed surface Pd2+ species when Pd loading is 0.5 mass% and below. Samples with higher loading preferentially contain nanosized Pd particles. Agglomeration of Pd species during thermal aging was found to take place starting from Pd concentration of 1.0 mass%. In some cases, size of Pd particles exceeds 150 nm, which is in about 40 times higher comparing with the initial samples. According to XPS data, degree of Pd2+-alumina interaction in the aged samples is also increased.