The nitrogen and sulphur chemistry is more significant during compression of flue gases than they are under atmospheric conditions. This fact became apparent during the development of oxy-fuel power plant technology to capture carbon dioxide (CO 2 ). In the oxy-fuel power plant, the CO 2 -rich flue gas stream is compressed to enable efficient transport and storage. During this process, NO x and SO x are removed as acids in the condensed water. However, the chemistry of these steps is not understood well enough to allow for control and design of the process.In the present work, the gas- and liquid-phase chemistry of NO x and SO x at elevated pressures were evaluated by comparing a state-of-the-art reaction mechanism to the results of experimental investigations. The model used confirms previous observations of substantial absorption of NO x and SO x and subsequent formation of acids in pressurised flue gas systems. The results of the modelling show that the oxidation of NO into NO 2 governs the absorption of NO x . The complex chemistry of the liquid phase, which includes reactions between HNO 2 , H 2 SO 3 , and possibly H 2 SO 4 , is critical for the rate of absorption of NO x and SO x from the gas to the liquid phase. This process is heavily dependent upon the pH level. The modelling suggests that N 2 O is formed as a stable product through the liquid-phase reactions.