Free nitrous acid (FNA) has been shown to enhance the biodegradability of waste activated sludge (WAS) but its effectiveness on the pre-treatment of mixed sludge is not known. This study explores the effectiveness of four different FNA concentrations (0, 2.49, 3.55, 4.62 mg N-HNO2/L) and three exposure times (2, 5, 9 h) lower than the ones reported in literature (24 h) on WAS characteristics and specific methane production (SMP). FNA pre-treatment reduced sludge cell viability below 10% in all cases after an exposure time of 5 h, increasing the solubility of the organic matter. The treated mixed sludge was used as substrate for the biochemical methane production tests to assess its SMP. Results showed a significant increase (up to 25%) on SMP when the sludge was pretreated with the lowest FNA concentration (2.49 mg N-HNO2/L) during 2 and 5 h but did not show any improvement at longer exposure times or higher FNA concentrations. (C) 2016 Elsevier Ltd. All rights reserved.