The content of polyphenolic substances in commercial onions has been determined. The antioxidant activity of their ethanolic extracts, as well as their effects on the oxidation of edible corn oil during accelerated ageing was studied. Maceration of taxonomically identified commercial vegetable samples, previously peeled and thinly sliced, was carried out at ambient temperature, out of direct light, with occasional agitation and ultrasound, employing 95% ethyl alcohol as the extraction solvent, allowing them to stand for 7 days. The total polyphenolic contents were determined on the filtrated extracts using the Folin-Ciocalteau method. The antioxidant activity was evaluated on emulsions of ethanolic extracts of onion prepared in edible commercial corn oil, using sorbitan monooleate as emulsifying agent. The peroxide values were analyzed using the iodometric method; oxidation induction times were obtained from the peroxide evolution graphs, using the tan- gent method. Oil samples emulsified with ethanolic onion extracts showed an extension of the induction period. A 7-day ageing study at 45?C was additionally performed to determine the conjugated dienes on pure commercial corn oil and its emulsions by visible spectrophotometry. The spectral analysis showed an increase of the measured absorbancies in oil samples without additives and no change for the oils emulsified with onion extract. An increasing of diene values was observed for corn oil without additives during ageing; no changes in the value were observed in oils emulsified with onion extracts.