Well-delaminated carboxymethylated nanofibrillated cellulose (NFCCarb) systems are prerequisites for many industrial applications. In this study it was shown that addition of water, in a narrow range, not only improves the efficiency of the carboxymethylation process, but also enhances the degree of delamination of NFCCarb, which leads to improved properties. The observations were proposed to be due to a more homogeneous distribution of the charged groups, brought about by the higher swelling of fibers with inclusion of water.