Abstract: The method of and a device for conditioning sludges, especially sewage sludges at an increased pressure and at an increased temperature wherein the raw sludge is fed by means of high-pressure pumps through a plurality of heat exchanging stations until it is brought in the last station to a pressure which is sufficient for the conditioning. Thereupon the conditioned sludge is returned through the same stations and in each station is pressure released and cooled. Steam released due to the cooling is brought in contact with the incoming raw sludge and warms up the same to a saturation temperature. To insure optimum heat exchange, each station is provided with means for measuring the concentration of gas components admixed to the steam and the discharge of the gaseous mixture from the station is controlled in response to the measured concentration value.