In the present dissertation garden rhubarb (Rheum rhaponticum) as well as rooibos tea (Aspalathus linearis) were reinvestigated for polyphenols. After extraction of plant materials, MLCCC and preparative HPLC were used for isolation of substances. The structures of these compounds were unequivocally verified by different NMR-experiments (1H, 13C, HMBC, HMQC) and HR-MS. From garden rhubarb stilbenes, stilbene-glycosides and flavonoids were isolated. In fermented and unfermented rooibos the following polyphenols were identified: flavanone-C-glycosides, dihydrochalcones, flavone-C-glycosides, flavonole-O-glycosides, hydroxy benzoic acids, hydroxycinnamic acids, flavones, flavonols and flavanols. The composition of polyphenols from unfermented rooibos tea is significantly altered by consecutive reactions of the dihydrochalcones during fermentation. The conversion of aspalathin to corresponding flavanone-C-glycosides and flavone-C-glycosides were verified by model incubations. The first important step during the formation of higher molecular browning products by aspalathin was characterized. The antioxidant potential of isolated flavonoids from rooibos tea was determined for the first time. Herein, three different assays were used (TEAC-assay, fremy's salt assay and LDLoxidation). The decoupled LDL-oxidation assay was used to approach the LDL-oxidation assay more related to the situation in vivo. The different assays (TEAC-assay, fremy's salt assay and LDL-oxidation) were further used for the development of a screening assay to determine the antioxidant capacity of compounds from unknown plant extracts before isolation of substances. Here, the different extracts of rooibos were separated by HPLC-DAD and fractionated. Each collected fraction was introduced into the different assays. The TEAC-assay was proven as the appropriate system for a screening assay.