The gasification process of phosphorus from municipal sewage sludge during carbothermic reduction process was investigated using a quadrupole mass spectrometer gas analysis and thermo-gravimetric analysis. Two types of sludge, with high and low Fe₂O₃ content, were used to evaluate the possible effect of Fe₂O₃ on the gasification behavior of phosphorus from sewage sludge during incineration processes. The gas species vaporized during the carbothermic reaction were measured by the quadrupole mass spectrometer with comparison to the ion currents for the respective molecular gas species. From the gas analysis, it was observed that PO and PO₂ gas species were dominantly vaporized around 1073 K. Metallic phase phosphorus gas of P₂ vaporized in the 1273–1700 K temperature range. The evolution of another metallic phase phosphorus gas of P₄ was found to be negligibly small. Fe₂O₃ in the sludge has an important role in the phosphorus emission from the sludge during carbothermic reduction reaction, since metallic phosphorous gases react with the reduced iron and form Fe–P alloy.