Abstract The antioxidant activity of 5-caffeoylquinic acid (5CQA) at concentration ranges of 0.01–0.08% (w/w) was compared with 0.02% (w/w) butylated hydroxytoluene (BHT) in soybean oil. The oil was stored in air at 55 °C and monitored by peroxide and conjugated diene measurements. Both 0.01% and 0.02% 5-CQA showed lower peroxide values than BHT only on the first day. However, on days 1, 2, 3 and 4, 0.04% 5-CQA reduced the peroxide value to 68%, 47%, 45% and 90% of that observed with BHT. Use of 5-CQA appears to delay the formation of peroxides and up to 0.04% (w / w) its antioxidative action increases with concentration for up to 3 days.