Nowadays the treatment of activated sludge often includes a thermal drying step. Data such as water vapour sorption isotherms and heat of sorption are required for good design and control of the drying processes. An equilibrium method was used to measure these data on domestic activated sludge, between 39 degreesC and 80 degreesC. The experimental investigation indicated that the nature of the sludge and its evolution (bulking sludge, anaerobic storage) ilo not affect the sorption isotherms. Experimental data were fitted to different well-known models; the Oswin relationship gave the better fit. Based on sorption data the heat of sorption as a function of moisture content was determined.