Simultaneous small- and wide-angle X-ray scattering was used to follow changes in the microstructure of native cellulose (cellulose I) fibers during conversion to sodium cellulose I (Na-cellulose I) by aqueous sodium hydroxide. Wide-angle X-ray scattering was used to monitor the extent of conversion, while small-angle X-ray scattering was used to explore what occurs at the higher structural levels of the elementary fibrils, microfibrils, and interfibrillar voids. Native cellulose fibers, swollen in either water or aqueous sodium hydroxide, exhibited an increase in the void volume fraction and a decrease in the void cross-section size, as the swelling agent separated elementary fibrils, opening up the structure, and creating many small voids. After conversion of swollen native cellulose to sodium cellulose I, the void volume fraction and average void cross-section dimensions both increased. During conversion from dry cellulose I fibers to swollen Na-cellulose I fibers, the void cross-section dimensions went through a minimum, suggesting that the void structure may go via an intermediate similar to the water swollen structure of cellulose I.