Carboxymethylation of mercerized cellulose was carried out in heterogeneous media by reaction with monochloroacetic acid at temperatures in the range of 30-70 degreesC. The time courses of monochloroacetic acid concentration, total degree of substitution (DS), mole fractions of mono-, di-, and trisubstituted anhydroglucose units, and partial DS at positions C2, C3, and C6 of the anhydroglucose units were measured. The presence of soluble decomposition products was assessed by analysis of the liquid phase. The kinetic interpretation of data was carried out by assuming negligible diffusional resistances and a bimolecular, nucleophilic substitution mechanism for carboxymethylation, with the participation of an additional parallel, pseudo-first-order, parasitic reaction leading to monochloroacetic acid decomposition. The proposed model provided a close interpretation of the experimental concentration profiles for both reagents and products.