It was found that the chemiluminescence (CL) produced from the reaction of luminol with iodine in the alkaline medium was strongly inhibited by plant phenolic compounds. Based on this finding, a new flow injection CL method was developed for the determination of caffeic acid and 6'-caffeoylerigeroside. The latter compound was isolated for the first time from Erigeron acris L. herb. The method was simple, rapid and sensitive with a detection limit of 4 x 10(-3) ng mL(-1) (caffeic acid) and 0.18 ng mL(-1) (6'-caffeoylerigeroside), linear range of 0.1-1.5 ng mL(-1) (caffeic acid) and 1-200 ng mL(-1) (6'-caffeoylerigeroside), relative standard deviation of 3.3% for 10 measurements of 0.45 ng mL(-1) caffeic acid and 2.9% for 40 ng mL(-1) 6'-caffeoylerigeroside. This method was successfully applied to determine the content of phenolic compounds/antioxidant activity of E. acris L. extracts and phenolic acids content in pharmaceutical formulation. A possible mechanism of the inhibition of the proposed CL system was discussed.