A process for the extracting soluble dietary fiber (SDF) from oat and barley grains uses enzymatic hydrolysis treatment. After milling the grain, any endosperm-depleted fractions rich in beta-glucans are recombined without further heat treatment, dispersed in water and then subjected to sequential enzymatic treatment with starch-degrading enzymes. Next is an optional step of enzyme inactivation by wet heat treatment, followed by a step wherein the hydrolyzate mixture is spontaneously or centrifugally separated into at least three distinct fractions. The first fraction comprises the SDF complex, containing more than 20% beta-glucan on a dry matter basis. Also, a second aqueous fraction and a third fraction contains most of the protein and oil together with the insoluble fibrous material from the milled grain.