The objective of the study was to determine the antioxidant properties of selected plant extracts from St John’s wort, buckwheat, hawthorn, helichrysum, as well as their important polyphenol constituents towards phosphatitylcholine liposome membranes (PC) oxidized with UVC radiation. Also, there were determined the antiradical properties of those substances towards the free radical 2,2-diphenyl-1-pikrylhydrazyl (DPPH^•) and the membrane association constants using a DPH (l,6-diphenyl-l,3,5-hexatrien) probe. There was confirmed the following relation of the antioxidant activities of the extracts, expressed as an IC₅₀^PC parameter: hawthorn – bark (21.69 mg/l) > St John’s wort (25.45 mg/l) > buckwheat – hull (30.33 mg/l) > hawthorn – leaves (32.52 mg/l) > buckwheat – haulm (37.47 mg/l) > helichrysum (115.06 mg/l). The antioxidant activity of selected polyphenol components changed as follows: quercetin (0.53 mg/l) > epicatechin (29.19 mg/l) > chlorogenic acid (62.59 mg/l) > rutin (78.97 mg/l). The positive correlation between the antioxidant activity of the extracts studied and phenolic compounds, and the antiradical activity (r≥0.94) indicates that the mechanism of their antioxidant activity consists in neutralizing free radicals. From among the flavonoids studied, the quercetin showed the highest antioxidant activity, a high antiradical activity (IC₅₀^DPPH• = 2.51 mg/l), and a high association constant towards the liposome membrane (K_a = 11.23 10³ l/mg). The quercetin molecules may deeply anchor into the liposome membrane, therefore, free radicals are obstructed when penetrating into the inside of the membrane; for that reason, the antioxidant effectiveness of the liposome membrane is enhanced.