Cellulose nanocrystals are commonly obtained by acid hydrolysis, particularly with H2SO4. However, a small amount of deposited sulfate-groups contributes to the degradation of their thermal stability. This study prepared thermally-stable and sulfate-group-free cellulose nanospheres (CNSs) from office waste paper by H2SO4 hydro-lysis followed by solvolytic desulfation. The optimal desulfation conditions (i.e., 5 wt% MeOH, reaction tem-perature of 90 ◦C, a reaction time of 20 min, 0.5 mM pyridine) were preliminarily found from a one-factor-at-a- time experiment and validated by the results of a central composite design. The optimal desulfation conditions promoted environmental sustainability with less pyridine and MeOH and comparably shorter reaction time. The desulfated CNSs had a significant thermal stability enhancement from 186 to 340 ◦C. Comprehensive charac-terization of the morphology, chemical composition, and thermal behavior of the desulfated CNSs reconfirmed the complete removal of sulfate groups without harmful pyridine residues, demonstrating the potential use of the thermally stable CNSs.