Dextrans partially functionalized with chloroacetate groups were obtained by reaction of dextran with chloroacetyl chloride using pyridine as catalyst and the dimethylformamide/LiCl system as solvent. The structure of the resulting polymers was determined by means of infra-red, H-1 and C-13 nuclear magnetic resonance (n.m.r.) spectroscopy. C-13 n.m.r. spectra at 75.4 MHz of partially modified dextran with chloroacetate groups were studied in order to evaluate the selectivity of the reaction of dextran with chloroacetyl chloride in the homogeneous phase. Analysis of the spectra of ring carbons in the anhydroglucose units shows that the reactivity of the individual hydroxyl groups decreases in the order C2>C3>C4. The coupling of model bioactive carboxylic acids (alpha-naphthylacetic and 6-methoxy-alpha-2-naphthaleneacetic (naproxen)) to dextran functionalized with chloroacetate groups was carried out by reaction with their potassium salts or directly in the presence of 1,8-diazabicyclo[5.4.0]-7-undecene. High degrees of modification were obtained by both methods. However, the esterification reaction is somewhat more efficient in the case of the potassium salts.