The imidazolium based ionic liquids as a new class of solvents for cellulose have been used for modification reactions under homogeneous conditions. Reactions of cellulose have been investigated in solutions in 1-butyl-3-methylimidazolium chloride (BMIMCl), acetate (BMIMOAc) and in 1-butyl-2,3-dimethylimidazolium chloride (BMMIMCl).A novel drying method for the imidazolium based ionic liquids has been developed. Water was removed with hexamethyldisilazane as silylating agent, which allowed to remove ammonia and hexamethyldisiloxane from imidazolium chlorides. Other imidazolium salts, in particular the carboxylates, could be purified by molecular distillation. Pure samples of nondistillable ionic liquids have been synthesized by anion exchange of distilled imidazolium acetates.Modification reactions of cellulose studied were acetalization, silylation, desilylation, carbanilation and hydrolytic degradation.Cellulose acetals have been synthesized from reactions of vinyl ethers with cellulose and cellulose derivatives. Distilled imidazolium acetates were very efficient reaction media for complete silylation of cellulose with hexamethyldisilazane. Desilylation of trimethylsilyl cellulose has been achieved with water alone, sterically demanding 1,1,2-trimethylpropylsilyl cellulose could be completely desilylated with potassium hydrogen fluoride in ionic liquids. Carbanilation of cellulose with phenyl isocyanate could be carried out in BMIMCl. Controlled degradation of cellulose at 150 °C in the imidazolium chlorides has been investigated and a hydrolytic mechanism has been proved.