Oxidation of primary hydroxyl groups of potato starch to the corresponding carboxylic acids was performed in a fluidized bed using N-2-diluted NO2, as oxidant. FT-IR spectrum of the oxidized starch show the presence of two absorption bands at 1743 and 1379 cm(-1). The absorption at 1743 cm(-1) which normally attributed to carbonyl absorption vanishes after washing the oxidized starch by water. Then two new absorption bands appeared at 1650 and 1400 cm(-1) corresponding to the ionized carboxyl groups. The presence of nitrate favor the un-ionized form of carboxyl groups and then the band at 1743 cm(-1) can be observed. Furthermore, the ionized carboxyl absorption (1600, 1400 cm(-1)) are replaced by carbonyl absorption at 1743 cm(-1) by adding DCl. By means of C-13 NMR analysis, oxidized starch, show a band around 200 ppm attributed to carboxylic acid, no formic acid was detected indicating that no cleavage took place under the applied reactions conditions.