Black teas are known for their characteristic brown colour, bitter taste and astringent mouth feel. These sensory characteristics are mainly influenced by the phenolic oxidation products present in black tea. The oxidation of phenolics from green tea leaves during black tea manufacturing is an uncontrolled process. With the objective to make tea oxidation a more controlled process, the aim of this thesis was to understand the enzymatic oxidation reactions occurring during tea oxidation, and to enable more rapid analysis of complex mixtures of phenolics. By incubating green tea catechins with an exogenous tyrosinase, a black tea-like phenolic profile was obtained, enriched in theaflavins, which are important for quality of tea. Further oxidation of theaflavins yielded theatridimensins, in which an epicatechin is coupled to the benzotropolone ring of theaflavin. By using MS/MS on selected ions these theatridimensins were shown to occur in black tea. This MS method could also be used to distinguish isomeric procyanidins and dehydrocatechins based on MS 2 fragments, as well as the different interflavanic configurations occurring in dehydrodicatechins. The dehydrocatechins were shown to occur in black tea as well. Besides these oligomerization reactions mediated by tyrosinase, oxidation of tea phenolics also comprised hydroxylation. The enzymatic activity from tea leaves responsible for this hydroxylation reaction, was found to be peroxidase. All findings were condensed into a new version of the 'oxidative cascade hypothesis', describing the oxidation reactions towards formation of a black tea.