Interference from phosphorus on sulphur capture during co-firing of sludge with wood has been investigated in a circulating fluidised bed boiler. Chemical equilibrium analyses were performed on the combustion system to complement the experimental results. It was found that the relatively high content of phosphorus in municipal sewage sludge interferes with the sulphur capture by occupying calcium, which otherwise would be available for reaction with sulphur. This fact must be taken into account when Sulphur capture strategies are decided for reduction of sulphur dioxide emissions from sewage sludge as an additional fuel. (c) 2006 Elsevier Ltd. All rights reserved.