Nonenzymatic oxidative degradation of aspalathin resulted in formation of the characteristic red-brown color of processed rooibos tea. Three colorless aspalathin dimers and dihydrocaffeic acid were first identified in model reactions. In addition, for the first time two colored dibenzofurans and their mechanistic precursors were isolated and unequivocally established. Color dilution analysis revealed these compounds as key chromophores. After trapping they were detected in fermented rooibos tea. Enzyme kinetic studies showed reversible inhibitory activities for the polyphenols aspalathin, (+)-catechin and isoquercitrin on α-glucosidase, α-amylase and sucrase. Aspalathin mainly turned out to be the most potent inhibitor. In in vivo tests in rats aspalathin was most effective in suppressing postprandial blood glucose levels. The results indicated a clear inhibitory potential of components of unfermented rooibos tea on enzymes of the gastrointestinal tract. Especially aspalathin may contribute to the control of hyperglycemia.