Laboratory tests of the reactivity of two limestones used for desulphurization of fluidized bed boilers showed that the desulphurization capacity of their calcinates do not yet correspond to routinely applied opinions on the absolute content of CaO in the limestone. Limestone with lower CaO content desulphurised the combustion gas more. This was caused by the fact that the desulphurization reactions of combustion gas are significantly negatively affected by the reaction of CaO with ballast oxides of ash and limestone. The amount of ballast oxides in both limestones rises along with the decrease of the particle size. When choosing proper limestone for the given power equipment it is necessary to respect these side effects. It is useful to pre-select and optimise the distribution the limestone particles according to the nature of the burnt coal. Thereby it is possible to reduce the consumption while complying with the SO2 emission limits. (C) 2012 Elsevier Ltd. All rights reserved.