Cotton cellulose was partially carboxymethylated by a one-bath method using monochloroacetic acid and sodium hydroxide. A method was developed to study the extent of the carboxymethylation of cellulose within the cotton fibers using scanning electron microscopy (SEM) and energy-dispersive X-ray analysis (EDX). Uranyl nitrate was used as a stain. Scanning electron photomicrographs revealed that the carboxymethylation of cellulose occurred throughout the cotton fiber cross section. However, the X-ray microanalysis data indicated a variation in the extent of the reaction among the morphological regions that have different chemical accessibility.