In the study, a series of methylimidazolium ionic liquids were prepared, including five acidic and seven neutral ionic liquids; and the effects of these ionic liquids on swelling behavior of bagasse cellulose were investigated. The focus of this work is to investigate the relationship between the properties of ionic liquids and the swelling capacity. The swelling absorbency for cellulose in the acidic ionic liquid aqueous solutions was less than in distilled water. It was found that the partial hydrolysis on cellulose surface occurred along with swelling simultaneously. In neutral ionic liquid aqueous solutions, the nonlinearity of swelling ability with the alkyl chain length was discovered. It could be attributed to the dual role of hydrophobic interactions and steric effects provided by cations. Furthermore, small polarizable anions also contribute to the swelling of cellulose to some extent. The properties of treated cellulose were also investigated and compared with the native fibers by scanning electron microscopy. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40199.