Formic acid was found to dissolve sodium carboxymethylcellulose (Na-CMC) with comparatively slight chain degradation. The dissolution of Na-CMC in a wide range of degree of substitution from 0.4 to 2.0 occurs under partial formylation of the remaining OH groups of the polymer, i.e., formic acid is the first non-aqueous, so-called derivatizing solvent. The Na-CMC formyl esters, which can be isolated under water-free conditions, are even soluble in dipolar-aprotic solvents, and they can be regenerated to Na-CMC by treating with aqueous solutions of NaOH.