The mathematical model of Kiil et al, (Ind. Eng, Chem. Res. 37 (1998) 2792) for a wet flue gas desulphurisation (FGD) pilot plant was extended to include the simultaneous absorption of HCl. In contrast to earlier models for wet FGD plants, the inclusion of population balance equations for the limestone particles enabled a quantitative description of the influence of HCl absorption on essential process parameters such as the degree of desulphurisation and the residual limestone level of the gypsum produced. Simulations showed that the presence of 100 ppmv HCl in the flue gas reduced the degree of desulphurisation from 85 to 84% and increased the residual limestone level of the gypsum from 2.1 to 2.4 wt%. It was found that these undesired effects from HCl absorption could be counteracted by adding adipic acid to the slurry in a concentration of about 1 mM. The influence of holding tank pH and the inlet flue gas concentration of SO2 on the degree of desulphurisation and the residual limestone level was found to be almost the same irrespective of HCl was present (100 ppmv) in the flue gas or not. The results presented are of importance in the analysis of the performance of wet FGD plants installed at power plants firing coals of varying Cl contents.