Disclosed is a process for producing water-insoluble cellulose fibers having high carboxyl content by reacting cellulose fiber in suspension with dicarboxylic acid anhydride or chloride in the presence of a basic catalyst. The fibers produced possess a unique combination of high carboxyl content in the range of 100 to 4000 meq/kg, and high average viscosity, in the range of 0.5 to 12 dl/g. The carboxylated cellulose fibers according the invention can be made with a combination of carboxyl content and average viscosity which are suitable for use in numerous applications, including absorbent products, health care products, specialty papers, adhesives, detergents, biodegradable fibers, precursors for aqueous coatings and ion exchange fibers.