Sodium carboxymethyl starch (CMS) Was prepared using an extruder as :a continuous solventless chemical reactor. The effects of NaOH, H2O, temperature, ethanol, sodium monochloro acetate (SMCA), sodium tripolyphosphate (STPP), Citric acid, epichlorohydrin, and screw configuration on the degree of substitution (DS) and swelling properties of CMS were studied. Using these factors, CMS with the DS required for :pharmaceutical disintegrant applications (0.22-035) was prepared By modifying the water/ethanol ratio in the formulations and avoiding excess gelatinization caused by use of NaOH solution, CMS with rapid swelling properties in an aqueous medium, essential for superdisintegrit applications in solid dosage forms, was obtained. Both-corn and potato starches were used to prepare the CMS. The carboxymethyl starch, with rapid swelling properties, was characterized using Fourier transform infrared spectroscopy and scanning electron microscopy.