The effect of inoculum acclimation on methane production from mesophilic digestion of microwave pretreated of thickened waste activated sludge (TWAS) was investigated. In a range of 50-175 degrees C, microwave disintegrated waste activated sludge and enhanced methane production. Soluble to total chemical oxygen demand ratios increased from 9 +/- 1 (control; unpretreated) to 24 +/- 3, 28 +/- 1 and 35 +/-1% at 120, 150 and 175 degrees C, respectively. Biological Methane Potential tests indicated that despite mild acute inhibition in the first 9 d, acclimated inoculum digesting sludge, irradiated to 175 degrees C produced 31 +/- 6% higher biogas compared to control after 18 d of digestion. However, initial acute inhibition was more severe for non-acclimatized inoculum requiring 2 times longer recovery with only 18 +/- 2% higher biogas after 20 d. Inoculum acclimation did not only accelerate production of biogas, but also increased the extent of the ultimate biodegradation of pretreated waste sludge.