Waste activated sludge (WAS) samples that were generated over a range of solids residence times (SRTs) were employed in bench scale ozonation tests to evaluate the impact of ozonation, on physical, chemical and biochemical properties of WAS and digestibility. Solubilization responses suggested that the types of solubilized materials were affected by the ozone dose and the SRT of WAS. The results obtained from biochemical methane potential (BMP) showed that ozonation did not considerably increase the ultimate digestibility of shorter SRT sludges while a high dose caused a substantial increase in the digestibility of a 15 day SRT sludge. The biochemical acid potential (BAP) tests as a shorter term test (10 days) than the BMP (55 days) test could provide information on hydrolysis and acidification/ammonification rates. The results revealed that ozonation substantially increased the rate of hydrolysis which is often the rate limiting process in WAS digestion. (C) 2010 Elsevier Ltd. All rights reserved.