Oxidation and reduction processes are fundamental to many of the proposed mechanisms by which dietary phytochemicals are thought to exert protective effects against cardiovascular disease and some cancers. An understanding of the redox chemistry of these compounds is essential in assessing their potential to participate in these processes. Phenylpropanoid-derived compounds were selected and synthesised where required to represent many of the structural features found in this important group of compounds. Using electron paramagnetic resonance spectroscopy and computational chemistry a structure–redox activity relationship was obtained. Good correlation of computational and experimental results was observed for the mono-hydroxylated compounds. This demonstrated the value of computational chemistry in obtaining information about compounds, not readily available and the effect of electron delocalisation on parent radical stability. For compounds containing more than one hydroxyl, the relationship was found to be more complex. The importance of quinone formation in compounds containing more than one hydroxyl substituent was highlighted, as this was found to have a significant effect on stabilisation and therefore, their participation in redox processes.