<p>The new cellulose solvent, ditetrabutylammonium hydrogen phosphate/dimethyl sulfoxide (TBA₂HPO₄/DMSO), can be used as reaction medium for the homogeneous derivatization of cellulose. Due to the excellent solubility, both homogenous esterification and etherification of cellulose can be carried out at mild temperature in concentrated cellulose solution. Cellulose acetate (CA) with the desired degree of substitution, ranging from 0.6 to 2.46, has been successfully prepared in TBA₂HPO₄/DMSO by varying the reaction parameters, including reaction time, temperature, the amount of acetic anhydride and the composition of solvents. The functional group distribution is consistent with that of homogenous reactions. The solubility of synthesized CA in some common solvents, such as DMSO, DMF, CHCl₃, THF, pyridine, acetone, and H₂O, depends on the degree of substitution (DS) and distribution of substitution. Meanwhile, due to the weak alkalinity of the solvent, the homogeneous etherification reaction can be easily carried out, without extra alkali addition. Hydroxypropyl cellulose (HPC) and carboxymethyl cellulose (CMC) can be synthesized in the homogeneous TBA₂HPO₄/DMSO system. The DS of HPC increases with the rising feeding epoxypropane/AGU ratio. CMC with higher DS can also be obtained with the addition of NaOH. The results in this work confirm that TBA₂HPO₄/DMSO solvent system is a promising medium for the homogenous esterification and etherification of cellulose, which would provide a new platform for further study of homogeneous cellulose reactions and functional cellulose materials.</p>