Cellulose sulfates (CSs), the sulfuric acid half ester of cellulose in their sodium form, are useful compounds for certain biomedical applications and for the formation of polyelectrolyte complex superstructures [66]. However, the homoge­neous synthesis of these derivatives in ILs proofed to be more challenging com­pared to the preparation of organic cellulose esters [33]. The same holds true for toluene sulfonic acid esters of cellulose, usually referred to as tosyl celluloses (TOSCs), that are versatile intermediates for the preparation of polysaccharide derivatives via nucleophilic displacement reaction [35]. Both cellulose derivatives are usually prepared at or below 25 °C to prevent side reactions. Sulfation at elevated temperature often induces severe polymer degradation whereas tosylation might yield chlorinated or cross-linked products. At these low temperatures, the exceptional high viscosity of cellulose/IL solutions (see Sect. 5.3.1) prevents efficient mixing during the reactions, which results in non-uniform product mix­tures. Interestingly, EMIMAc, a cellulose dissolving IL with rather low viscosity, cannot be applied as reaction medium. The acetate anion, present in high concen­trations, forms mixed anhydrides with the tosylation and sulfation reagents and acetatylation instead of formation of the desired cellulose derivatives occurs (see Sect. 5.3.2) [67]. Certain co-solvents, such as DMSO, DMF, pyridine, or N, N — ‘-dimethylimidazolidinone (DMI) can be added to cellulose/IL solutions to dimin­ish viscosity (see Sect. 5.3.1) [33, 68, 69]. Thus, completely homogeneous sulfation and tosylation of cellulose could be achieved at 25 °C with little polymer degrada­tion using BMIMCl/co-solvent mixtures as reaction media [33, 35]. CSs prepared by this procedure exhibit good water solubility even at low DS values of 0.2-0.3. The derivatization reaction shows higher regioselectivity compared to other het­erogeneous or quasi-homogeneous methods for sulfation of cellulose; at DS < 1 the vast majority of sulfate groups are located at position 6. Moreover, the DS and consequently the product properties can be tailored by adjusting the molar ratio AGU to sulfation reagent (Fig. 5.3). In a comparable approach, TOSC with DStosyi < 1.14 have been prepared by homogeneous derivatization of cellulose in BMIMCl/pyridine and BMIMCl/DMI mixtures [35]. The amount of deoxy-chloro moieties introduced as well as the extent of polymer degradation can be controlled by adjusting the reaction time from 4 to 8 h.