The direct esterification reaction of dextran with alpha-naphthylacetic acid (a model bioactive carboxylic acid) is studied in homogeneous phase using pyridine-sulfonyl chloride as catalyst. The structure of the resulting adducts was determined by means of infra-red, H-1 nuclear magnetic resonance (n.m.r.) and C-13 n.m.r. spectroscopy. The influence of the pyridine concentration, the type of the sulfonyl chloride as well as the temperature was evaluated. Direct esterification reaction was also performed with other bioactive carboxylic acids, such as naproxen and nicotinic acid. C-13 n.m.r. spectra at 75.4 MHz of partially modified dextran with alpha-naphthylacetate groups were studied in order to evaluate the selectivity of the reaction between dextran and alpha-naphthylacetic acid. 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 > C4 > C3. On the basis of these results a probable mechanism for the reaction is suggested.