The viscosity of suspensions depends strongly on the properties of the applied particles like particle size, particle size distribution, and solid load and especially on the specific surface area. Carboxymethelcellulose is an anionic water soluble polymer. CMC HV is a high viscosity carboxymethylcellulose which is used as a viscosifier and fluid loss reducing agent in different types of water based drilling fluids. The micronization of carboxymethylcellulose using the rapid expansion of supercritical solution CRESS) process was investigated in this study. The effects of the operation parameters for the RESS process were discussed by design of experiment (DOE) for the extraction temperature (35-55 degrees C), extraction pressure (150-200 bar), nozzle diameter (500-1200 mu m), spraying distance (1-5 cm) and cosolvent (deionized water, isopropanol). Results from analysis of variance (ANOVA) showed that extraction pressure and cosolvent were the most significant factors, and using deionized water favored the production of smaller particles as cosolvent. The CMC particles were micronized from the original average size of 15 mu m to the smallest average size of 0.14 mu m through the RESS process in this study. The optimal operation condition for preparing CMC micro-particles in the range of this work is: the pressure of 200 bar, the temperature of 55 degrees C, with 500 mu m nozzle and 1 cm spraying distance in the presence of deionized water as cosolvent Smaller particles were produced with increasing extraction pressure and temperature. In addition, the smaller the nozzle diameter, the smaller the particles and the narrower the PSD obtained. (C) 2012 Elsevier B.V. All rights reserved.