Combustion of wheat straw for heat and power production is often challenging due to ash related problems such as fouling deposition and high temperature corrosion. Release and transformation of ash forming elements K, Cl and S directly respond to these problems. In this work, sewage sludge was tested as an additive to reduce potassium release during the wheat straw combustion. The wheat straw with and without sewage sludge addition (5 %, wt/wt) were combusted in a temperature range 700 °C to 1000 °C. The ash residues from each combustion test were analyzed with a combination of an inductively coupled plasma optical emission spectroscopy (ICP-OES), scanning electron microscopy equipped with energy dispersive X-Ray analysis (SEM-EDX) and X-Ray diffractonmetry (XRD). It was found that that release of potassium from the wheat straw started already at 700 °C. About 80 % of the potassium released from after the wheat straw combusted at 1,000 °C for 1 h. Most of the potassium evaporated as potassium chloride in the temperature range 700-900 °C as indicated by SEM-EDX and XRD analyses. Addition of sewage sludge considerably reduced potassium release from the wheat straw as temperature was over 800 °C. At 1000 °C, less than 60 % potassium released from the wheat straw and the rest retained in ash residues after combustion tests. The SEM-EDX and XRD analysis results revealed that aluminum silicates, phosphorus and calcium in the sewage sludge played critical roles in capturing potassium from the wheat straw, through forming different potassium aluminum silicates and phosphates. The results from current study indicate that addition of sewage sludge may be beneficial to reduce potassium release and thereby fouling deposits in wheat straw combustion applications.