The present invention provides an .alpha.-glycosyl isoquercitrin-containing novel composition which has a high in vivo absorbability, and hence exhibits a significant in vivo antioxidative activity. The present invention further provides preparation methods for such a composition. The composition contains a mixture of quercetin glycosides represented by the following formula: ##STR00001## wherein Glc represents a glucose residue, and n is 0 or a positive integer of 1 or more, includes at least a quercetin glycoside wherein n is 3, and satisfies the following requirement (a): (a) the total proportion of quercetin glycosides in which n is 3, and in which other n values may be 1 or 2, or 1 and 2, is 50 mol % or more, and the total proportion of quercetin glycosides wherein n is 4 or more is 15 mol % or less, in the composition. The composition can be prepared by treating an enzymatically modified isoquercitrin with .beta.-amylase.