Dissolution of microcrystalline cellulose (MCC) in pure ionic liquids (ILs) and IL/dimethyl sulfoxide (DMSO) mixtures (mole fraction χ DMSO = 0.2–0.9) was quantified using a specially constructed mechanical stirring system that allows reproducible agitation speed; temperature control, and minimum solution-air contact. The electrolytes employed were: 1-( n -butyl)-3-methylimidazolium acetate (C 4 MeIm AcO), 1-(methoxyethyl)-3-methylimidazolium acetate (C 3 OMeIm AcO), 1,8-diazabicyclo[5.4.0]undec-7-enium acetate (DBU AcO), tetramethylguanidinium acetate (TMG AcO), and tetra( n- butyl)ammonium fluoride hydrate (TBAF·xH 2 O). The effects on MCC dissolution of IL/DMSO composition, and temperature (50, 70°C) were studied. C 4 MeIm AcO and C 4 MeIm AcO/DMSO were more efficient solvents than their C 3 OMeIm AcO counterparts, due to “deactivation” of the ether oxygen of C 3 OMeIm AcO. MCC dissolution by C 4 MeIm AcO/DMSO was compared with DBU AcO/DMSO, TMG AcO/DMSO at χ DMSO = 0.6, and TBAF·xH 2 O/DMSO at χ DMSO = 0.95. The relative efficiency was (solutions in DMSO): C 4 MeIm AcO > C 3 OMeIm AcO > DBU AcO > TMG AcO > TBAF·xH 2 O. The efficiency of C 4 MeIm AcO relative to C 3 OMeIm AcO is due to higher solution basicity. Isothermal titration calorimetry was used to study cellobiose-solvent interactions. Except for TBAF·xH 2 O/DMSO, these interactions are exothermic; the relative solvent efficiency increases with increasing dissolution |enthalpy|. Using the mole fraction concentration scale to report cellulose dissolution avoids possible ambiguities.