It is well known that Pd based catalyst deactivate during formic-acid decomposition in aqueous phase at mild temperatures. This study reports on a kinetic study of formic acid decomposition over Pd/gamma-Al2O3 catalysts including the effect of traces of oxygen, as well as pretreatment of the catalysts and supported by in-situ Attenuated Total Reflection Infrared Spectroscopy experiments. The results show that deactivation of Pd/gamma-Al2O3 catalysts can be suppressed by adding traces of oxygen. This is assigned to removal of adsorbed CO, poisoning the Pd surface, via oxidation to CO2. The activity of the catalyst during operation is maintained, promoting the H-2 production compared to operation in absence of any oxygen. Clearly, oxygen oxidizes CO preferentially over H-2 under the condition that the oxygen concentration is kept below 0.1 vol% in this study. Further increasing the oxygen concentration further increases conversion rate of formic-acid but also decreases the hydrogen yield significantly because formic acid oxidation and/or consecutive H-2 oxidation become dominating. The results of this study are important because the effect of traces of oxygen from ambient has not been considered in most of the reports in literature. (c) 2020 The Author(s). Published by Elsevier Inc. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).