BACKGROUND The water unextractable arabinoxylans (WUAX) contain beneficial phenolic compounds that can be used for food rather than for animal feed. The antioxidant activities of defatted rice bran obtained by xylanase‐aided extraction is reported herein. The chemical and molecular characteristics of extracted fractions were investigated. RESULTS The WUAX hydrolysate precipitated by 0–60% ethanol (F60), 60–90% ethanol (F6090), and more than 90% ethanol (F90) had decreased molar masses with increasing ethanol concentration. The fractions of interest, F60 and F6090, contained 75% arabinoxylans with ferulic acid as the major bound phenolic acid, followed by p‐coumaric acid. According to chemical‐based antioxidant assays F60 and F6090 exhibited higher diphenyl‐1‐picrylhydrazyl (DPPH) radical scavenging and ferric iron reducing ability than F90 which contained minor contents of small sugars and free phenolic acids. In cell‐based antioxidant assays, using the fluorescent 2′,7′‐dichlorofluorescein diacetate probe, all three fractions were potent intracellular scavengers. CONCLUSION The high molar mass of WUAX hydrolysates with high amount of bound phenolics contributes to the chemical‐based antioxidant activity. All fractions of WUAX hydrolysates showed high potent intracellular scavenging activity regardless of molar mass, content and the component of bound phenolics. © 2017 Society of Chemical Industry