Mercury displays a different reactivity and behaviour depending on its speciation. Determination of the mercury species present in combustion flue gases is important for proposing effective control technologies. The Ontario Hydro (OH) method is accepted as the only wet-chemical method suitable for measuring total and speciated mercury in flue gases. However, the continuous development of combustion technologies has the effect of modifying the operational variables and the composition of the resulting flue gases, leading to measurement biases in mercury determination. In this work, an alternative method based on the use of an ionic exchanger resin is proposed for the determination of gaseous oxidised mercury. The results show that for sampling flue gases containing reactive gases over long periods of time, the use of this resin is a suitable method for determining gaseous elemental and oxidised mercury, even in the presence of large amounts SO2 or NO2. Application of the OH method when elevated amounts of SO2 or NO2 are present in the gas composition leads to an overestimation of the amount of oxidised mercury due to the oxidation of the mercury in the KCl impinger solution. (C) 2014 Elsevier Ltd. All rights reserved.