Periodate oxidation is a selective method of introducing aldehyde groups into cellulose, allowing further functionalization and modification. This reaction occurs primarily at the interface between amorphous and crystalline regions, with crystallinity playing a crucial role in determining the reaction progress. In this study, cellulose was dissolved in cold alkali and reprecipitated with acid to produce a highly swollen structure, thereby improving fiber accessibility. The degree of swelling achieved, which is inversely related to crystallinity, increased the fiber surface area and improved reactivity by exposing more accessible sites for oxidation. The degree of substitution (DS) was calculated and showed a gradual increase in aldehyde content.In addition, the highly swollen cellulose structure retained significant amounts of water, which, upon drying, led to severe hornification, limiting the accessibility of reactive sites. This loss of accessibility negatively affected the reactivity of the samples during oxidation. To address this challenge, different drying methods—including air drying, freeze drying, and glycerol drying—were evaluated for their effectiveness in minimizing hornification and maintaining cellulose reactivity. The resulting products were characterized using Fourier transform infrared spectroscopy (FTIR) and polarized-light optical microscopy, and oxidation levels were quantitatively analyzed. These findings contribute to the optimization of cellulose modification strategies and facilitate the development of high-performance cellulose intermediates for further functionalization.In summary, swelling of cellulose in cold alkali strongly stimulated the periodate oxidation especially for the never dried samples.