Carboxylic acid groups were functionalized to MWNTs using some aqueous acid solutions and the resultant carboxylated MWNTs were in detail analyzed through structural characterizations. The characteristic Bragg peaks of pristine MWNTs were analyzed by XRD measurements. After acid treatment the diffraction peaks (100), (101), and (102) of pristine MWNTs disappeared, but the diffraction peak (002) was maintained in the carboxylated MWNTs. The structural and chemical changes after carboxylation of MWNTs improved the dispersibility of the resultant carboxylated MWNTs in water. (C) 2007 Elsevier B.V. All rights reserved.