The present state of understanding of sulphur capture in fluidized-bed combustion is discussed with focus on the possibilities to predict sulphur capture performance and the effect of intermittent reducing conditions. The four key factors that determine the sulphur capture performance are: (i) particle size distribution, including the effect of particle size reduction, (ii) residence time as a function of particle size, (iii) reactivity as a function of particle size, and (iv) effect of reducing conditions. The first three of these can be determined and included in a model for prediction of the sulphur capture, but the problem of how to include the effect of reducing conditions in a model is still unresolved. The conclusion must be made that sulphur capture performance in CFBBs cannot safely be predicted at present, and more research is needed before the effect of reducing conditions can be incorporated in modelling. Nevertheless, existing models can be useful to approximate the effects of, for instance, particle size reduction or changes in particle residence time.