The effect of phosphorus on the behavior of potassium and the reaction mechanism between potassium and phosphorus during the co-combustion of wheat straw (WS) and municipal sewage sludge (MSS) are investigated in a bench-scale reactor. The elemental composition of ash is analyzed by inductively coupled plasma-optical emission spectroscopy (ICP-OES), and the morphology and crystalline phase are analyzed by scanning electron microscopy (SEM), and powder X-ray diffraction ()CRD). The results indicate that the addition of MSS to WS enhances potassium retention in ash due to the formation of high-melting-point compounds such as potassium aluminosilicates (e.g., KAlSi2O6 and KAlSi3O8) and alkali phosphates (e.g., Ca9MgK(PO4)(7)). Calcium phosphate (Ca-3(PO4)(2)) is selected as a model phosphorus compound in municipal sewage sludge ash to study the phosphorus chemistry. The addition of Ca-3(PO4)(2) is effective in preventing sintering and fusion of wheat straw ash. Ca-3(PO4)(2) not only dilutes the ash but also absorbs and reacts with potassium compounds to form Ca10K(PO4)(7). Further reaction mechanism of the phosphorus and potassium is investigated through the reaction between Ca-3(PO4)(2) and KCl, which produces Ca10K(PO4)(7) and Ca-5(PO4)(3)Cl at similar to 800 degrees C and higher temperatures, revealing the mechanism of potassium phosphate formation during the co-combustion of MSS and WS.