Abstract The main purpose of this paper is the study of elemental mercury adsorption phenomena on a commercially available activated carbon in different oxygen/nitrogen streams. A laboratory-scale apparatus with a fixed bed was used to adsorb elemental mercury vapors in an oxygen/nitrogen stream (synthetic gas). Temperature was varied from 90 to 150 °C. The inlet gas stream was a nitrogen/oxygen mixture with oxygen partial pressures varying from 4.9 to 99.3 kPa, in this gas stream elemental mercury vapors was added spanning the mercury concentration from 0.3 to 3 mg/m 3 . A commercial activated carbon (Darco G60 from BDH) was utilized as sorbent. The experiments led to the determination of the breakthrough curves for the fixed bed. Saturation data allow determining the adsorption isotherms for the temperatures tested, at the different oxygen partial pressures investigated. Experiments showed that the presence of oxygen in the gas phase increases the adsorption capacity of the sorbent and the higher the oxygen concentration the higher the adsorption capacity; however an unusual trend of the adsorption capacity as a function of the temperature was observed, suggesting the occurrence of different phenomena. In order to explain this trend a set of ad hoc experiments was realized pre-treating the carbon in oxygen/nitrogen gas phase and performing the adsorption of mercury in a nitrogen gas phase. These experiments allow the definition of a new model that accounts for the phenomena occurring during mercury adsorption in gas phase containing oxygen. Eventually, the model results well fit the experimental findings.