Cereal brans, the main by-products of grain milling for white flour production, represent 13-17% of the grain weight and are rich in dietary fiber (DF), minerals, and bioactive compounds. Bran by-products are still utilized in low value applications such as animal feed or bioethanol manufacture . However, their use in whole foods is limited by the presence of antinutrients (e.g., phytic acid (PA)) and high recalcitrant and insoluble DF, which negatively affect the hydration and rheological properties of other biopolymers in the food matrix, leading to deleterious texture and sensory perception. Traditional physical treatments like particle size reduction or high-shear extrusion have been utilized to improve bran functionality, but neither sufficiently enhancing fiber solubility or reducing PA levels, due to their high thermal stability. Subcritical water (SW) treatment, which uses controlled pressure and temperature for biomass hydrolysis at decentralized locations, offers a promising green alternative to boost soluble DF and reduce antinutrients. At subcritical water conditions (> 5MPa, 100-374 ºC) the presence of hot pressurized liquid water would allow for fast hydrolysis reactions of bran biopolymers, with high efficiency. Therefore, this work aims to understand the effect of various SW treatments on the composition and functionality of bran fractions from rye, wheat, and spelt sources (Figure 1). Bran fractions were treated in a continuous hydrolysis plant with SW at three different temperatures (200, 270, and 340 °C) and constant pressure (17 MPa). Following these SW treatments, treated brans were filtered and fractionated into soluble and insoluble fractions. The brans were compositionally characterized, including polysaccharide and mineral composition, PA, and starch content as well as structurally analyzed using microscopic, chromatographic and FTIR tools. For functionality, foaming, water absorption, viscosity and solubility were evaluated. Increasing SW temperature resulted in a higher ratio of soluble to insoluble fiber fraction for the three cereal sources, especially for wheat brans. Filtration after SW treatment removed small-molecular weight compounds (e.g., sugars) and depleted starch in the insoluble fraction, particularly at 340 ºC, concentrating these compounds in the soluble fraction. Insoluble fractions exhibited higher fiber (~75%) and protein (9-12%) concentrations than control brans. FTIR and chromatographic analyses also suggested that both proteins and cell-wall polysaccharides (i.e., arabinoxylans) suffered from hydrolysis due to the SW treatment. Regarding microstructure, control brans had fibrillar and compact heterogeneous particles, while the insoluble fractions had less compact, honeycomb-like structures. The soluble fractions showed smaller and spherical particles with concavities, resulting from spray-drying. Mineral analyses revealed higher potassium and phosphorus contents in the soluble fractions, the latter in agreement with PA hydrolysis/solubilization. PA levels were lower in treated samples than controls, with higher SW temperatures further reducing PA content. Regarding functionality, insoluble fractions absorbed more water due to arabinoxylan solubilization, enhanced at higher temperatures, while viscosity decreased due to starch removal and dextrinization. Foaming capacity improved, especially at higher temperatures, in the soluble faction, but stability was negatively affected. Overall, results demonstrated that SW treatment leads to an effective fiber solubilization, starch depletion, and reduced antinutrients, producing a high fiber fraction with improved water absorption and low viscosity, and contributing to their integration in the food matrix for designing fiber-rich baked food applications.