Carboxylated, mercerized cellulose fiber having at least 12 meq/100 g of carboxyl substitution at the C-6 position of the anhydroglucose units, which includes at least 20% cellulose II, may be produced by catalytically carboxylating mercerized cellulose fiber in at least two catalytic carboxylation stages in a continuous series in which the pH is adjusted, and a primary oxoammonium salt catalyst (or precursor thereof) and secondary oxidant is added, at the beginning of each stage. Optionally, the carboxylated, mercerized cellulose may be made substantially free of aldehyde groups by means of a stabilizing step following the final carboxylation stage, in which aldehyde groups present in the fibers are converted to carboxyl groups.