Rheological properties of cellulose dissolved in two ionic liquids (ILs), 1-allyl-3-methylimidazolium chloride (AmimCl) and 1-butyl-3-methylimidazolium chloride (BmimCl), with co-solvent dimethylsulfoxide (DMSO), are studied in the concentration range of cellulose from 0.070 to 6.0 wt%. The viscosities of ILs are exponentially decreased by adding DMSO in the concentration range of 0-100 wt%. The co-solvent DMSO decreases the monomer friction coefficient in cellulose solutions and has no significant change for the entanglement state of cellulose, thus results in the reduced solution viscosity, shortened relaxation time and unchanged moduli of the cross-over point. For cellulose solutions, dilute regime, semidilute unentangled regime and semidilute entangled regime were determined by steady shear experiments. In semidilute entangled regime, the specific viscosities eta(sp), relaxation time tau, and plateau modulus G(N), exhibit concentration dependences as eta(sp) similar to C-4.4, tau similar to C-2.2, andG(N) similar to C-1.9, respectively, in AmimCI-DMSO (80/20 w/w); and eta(sp) similar to C-4.3, tau similar to C-2.0, and G(N) similar to C-2.1, respectively, in BmimCI DMSO (80/20 w/w). Therefore, the rheological properties of cellulose/IL/DMSO solutions are approximately of IL-independence in this study. The dependence of eta(sp) upon cellulose concentration shows that the IL-DMSO mixture is more like a theta solvent for cellulose, and the thermodynamic properties of IL-DMSO mixtures are similar with those of ILs for cellulose at 25 degrees C. The conformation of cellulose in ILs would not be changed with the addition of DMSO not only in the dilute regime but also in the entanglement regime. (C) 2012 Elsevier Ltd. All rights reserved.