We have studied the activity and stability of EEL-fabricated Pt/CeOx and Pt/SiO2 supported model catalysts during catalytic reactions. Different mixtures of H-2 + O-2 diluted in argon and CO + O-2 diluted in argon have been used in the temperature range 300-475 degreesC. Reaction-induced morphology changes are observed for both types of samples after heat treatments in mixtures of H-2 + O-2 diluted in argon. The EEL-fabricated samples are stable for reproducible CO oxidation measurements for at least a period of 50 h at 400 degreesC. Pre-reduction at 400 degreesC is shown to be favorable for CO oxidation in rich mixtures for the EEL-fabricated Pt/CeOx catalyst, while the opposite is measured for the EEL-fabricated Pt/SiO2 catalyst. Two powder samples, Pt/CeOx and Pt/SiO2, have also been tested during similar pre-treatment studies. Increased activity for CO oxidation (rich conditions) is found for both types of samples after pre-reduction, compared with pre-oxidation.