Fibre sludge is a cellulose-rich by-product obtained from the chemical pulping process of wood chips. In this study, a one-step pretreatment procedure is considered in which fibre sludge can be simultaneously dissolved and depolymerised in an ionic liquid media. Task-specific ionic liquid (TSIL), i.e. 1-(4-sulfonylbutyl)-3-methylimidazolium chloride [SBMIM] Cl, is used as an ionic liquid media. Based on the results, [SBMIM]Cl has the ability to dissolve and hydrolyse fibre sludge in a one-step reaction. Yields of total reducing sugars (TRS) after [SBMIM]Cl pretreatment were not high (a mass fraction around 4-5%), but still comparable to yields of TRS after acid hydrolysis (a mass fraction between 2 and 8%). Furthermore, [SBMIM]Cl is able to dissolve and depolymerise both dry and wet material even up to a moisture mass fraction of 50%, which is a significant advantage when wet fibre sludge is used as a raw material. (C) 2013 Elsevier Ltd. All rights reserved.