Reducing the particle size of AACC wheat bran decreased water-holding capacity, due, in part, to the collapse of its fiber matrix. Physical structure of commercial oat bran collapsed during manufacture and further grinding had limited effect. Physical structure of microcrystalline celluloses and guar gums did not differ between product types. The amount of water held by oat bran and microcrystalline cellulose was increased by reducing particle size. Water binding properties of guar gum were not affected. Physical structure of dietary fiber must be considered if water-binding properties are to be useful predictors of stool bulking efficiency.