The gasifier represents an attractive alternative to the well-established thermal treatment system for municipal sewage sludge, but special attention should be paid to the high phosphorus content in sewage sludge. Phosphorus volatilization during the co-gasification process of municipal sewage sludge and coal mixtures was investigated using a laboratory-scale high-frequency furnace. The transformation of phosphorus-containing compounds in slag at different temperatures was also studied. The results indicate that the volatilization ratio of phosphorus monotonically increases with the increasing gasification temperature and the volatilization of phosphorus mainly takes place during the pyrolysis process. The organophosphorus compound makes up the main part of volatized phosphorus when the pyrolysis temperature is not higher than 1100 degrees C. Inorganic phosphorus does not volatilize obviously until 1200 degrees C. After gasification, most phosphorus in mixtures deposits in slag in the form of phosphorus-containing glass.