Abstract Arsenic emissions are currently considered to be one of foremost importance. Arsenic volatility is higher than most of trace elements, but its vaporization behaviour is strongly dependent on the atmosphere composition. In this sense, thermodynamic equilibrium calculations, using HSC-Chemistry 5.0 software, were performed to evaluate the influence of different compounds in the distribution and mode of occurrence of arsenic in co-combustion processes. The influence of different parameters influencing arsenic behaviour, such as temperature, pressure, trace element concentration and flue gas composition on equilibrium composition were also evaluated. Predicting arsenic species, based on combustion conditions and fuel composition, will be useful to choose the best available control technology to reduce arsenic emissions. Finally, the possible interactions between arsenic and different trace elements (TE), mercury, cadmium and antimony, relevant from an environmental point of view, have also been studied; these interactions are not usually considered in thermodynamic studies; however, TE’s interactions affects the behaviour of a single TE, not only as a result of the formation of new species, but also, because of the different reactivity of TEs towards different elements which may affect TE’s volatilization behaviours. From results obtained in this study it may be concluded that in most cases, arsenic is mainly captured in ashes as a result of the formation of thermally stable species both from interactions with bulk ash and TE’s interactions. Nevertheless, the presence of some compounds (silicon, chlorine and sulphur) may enhance arsenic volatilization.