<p>X-ray tomography is a powerful non-destructive technique for visualizing cellulosic fibers, paper products, and natural-fiber composites in three dimensions. However, visualizing internal structures remains challenging when the matrix and fibers have similar elemental compositions and densities due to low X-ray attenuation contrast. In this study, we optimized iron oxide nanoparticle (NP) labeling as a fiber-selective contrast strategy. A 2³ full-factorial design of experiments (DOE) approach was used to evaluate three variables in iron oxide synthesis: mole fraction of Fe²⁺ (i.e., mol Fe²⁺/(mol Fe²⁺ + mol Fe³⁺)), stirring speed, and impregnation time, and the response was iron loading measured by elemental analysis. Decreasing the Fe²⁺ mole fraction from 1 to 0.35 significantly increased the iron loading, allowing a twofold improvement compared to earlier labeling protocols, when coupled with shortening the impregnation time from 180 to 30&#xa0;min and decreasing the stirring speed. The lowest Fe²⁺ mole fraction (0.35) yielded isotropic iron oxide NPs characteristic of magnetite, which formed a uniform coating on the cellulosic fibers. The application of the optimized labeling protocol to different cellulosic fiber types (BCTMP, NBSK, delignified BCTMP, and cellulose filaments) revealed that the labeling efficiency improved most significantly in the absence of lignin and for higher specific surface area fibers. As a proof of concept, labeled fibers were embedded in an epoxy resin demonstrating the stability of the iron oxide NP coating and enabling improved visualization of both long pulp fibers and cellulose filaments in composites. These findings establish new relationships between iron oxide NP synthesis parameters, fiber composition and structure, and labeling outcomes, supporting iron oxide NP labeled fibers as robust tracers for X-ray tomography of cellulosic fiber networks in composites and related bioproducts.</p> Graphical abstract <p></p>