The pyrolysis of surplus activated sludge was investigated experimentally by means of a batch reaction furnace. The pyrolysis temperatures were varied from 400 to 1, 200°C. The effect of pyrolysis temperatures on the yields of produced gas, liquor, tar and char as well as the their constituents was studied. As a result, the following conclusions were drawn for the range of experimental conditions employed : 1) Pyrolysis of the activated sludge starts to occur at around 400°C. 2) Total gas yield increases approximately proportionally to the square of pyrolysis temperature, and main constituents of gas are H2, CO, CH4 and CO2. The heating value of gas produced at temperatures above 1, 000°C is about 4, 000 kcal/ Nm3. 3) The maximum yield of tar is attained at between 600 and 800°C. 4) At above 600°C, the char yield is scarcely affected by pyrolysis temperature, and is kept at a constant value of about 48 wt% (dry sludge basis). The heating value of char is between 1, 650 and 2, 200 kcal/kg-char. © 1980, Society of Chemical Engineers, Japan. All rights reserved.