Imidazolium-based ionic liquids (ILs), including 1-butyl-3-methylimidazolium chloride (BmimCl) and 1-allyl-3-methylimidazolium chloride (AmimCl), were used to improve the refining properties of pulps under mild conditions. Results showed that the macro appearance of pulps was virtually unchanged after pretreatment with ILs under mild conditions. In addition, chemical pulps pretreated with ILs are more suitable for pretreatment than chemimechanical pulps. Pretreatment with ILs facilitates the refining process by destroying hydrogen bonds, which exist extensively in fiber. At the same refining energy input, the Canada standard freeness (CSF) of refined pulp decreased with improved fiber quality. At identical CSF values, the energy consumed during refining significantly decreased because ILs pretreatment facilitated swelling and fibrillation. The viscosity of cellulose changed slightly after IL pretreatment. However, the crystallinity index and fines content decreased, and the micro appearance of the fiber surface changed. BmimCl has a stronger swelling influence on pulp than AmimCl under optimal conditions. (C) 2012 Elsevier Ltd. All rights reserved.