Quercetin is a natural compound widely distributed in the plant food. It is still not certain which forms of quercetin, aglycone or glycosides, are better bioavailable. In this study, for the first time, the bioavailability of quercetin and quercetin β-glucosides was compared from dietary sources. As a source of quercetin glucosides, two outer fleshy onion scales of Błońska variety containing 21.30 mg Q/g d.m. (99.3% of quercetin as glucosides) were used, while quercetin aglycone originated from onion dry skin of the same variety containing 34.80 mg Q/g d.m. (53.2 % of quercetin as aglycone). The results showed that bioavailability of quercetin glucosides was 50% of that of quercetin aglycone (p < 0.05). A factor which may be the reason for differences in the determined bioavailabilities are the mechanisms of transportation and hydrolysis. It is suggested that more hydrophobic quercetin may pass through biological membranes to reach intestinal enterocytes via passive transport, while for more polar quercetin glucosides two mechanisms are proposed: prior to absorption they may be hydrolyzed in the intestinal lumen by β-glucosidases and released free quercetin is absorbed as above, or they may enter enterocytes via Na+/glucose transporter.