Guarana (Paullinia cupana) is a popular herb native to the Amazon Basin and used extensively in soft drinks in Brazil, other Latin American countries, and more recently in the United States. Extracts derived from the dried seeds of guarana possess strong anti-platelet aggregatory properties. In this study, an active fraction containing this activity was purified and analyzed by high-performance liquid chromatography/mass spectrometry (HPLC/MS) techniques. It was noted that this fraction contains catechins, epicatechins, and their dimers, with a small amount of caffeine. It is suggested that complexes containing caffeine and catechins (and their dimers) might be responsible for anti-platelet aggregatory activity in guarana seeds and might offer health benefits towards decreasing risk of thrombosis and cardiovascular disease.