Free radical generation as a result of oxidation reactions of rosmarinic acid (RA), a caffeic acid ester with 3,4-dihydroxyphenyllactate, was investigated by electron paramagnetic resonance (EPR) spectroscopy using a variety of oxidation conditions. Limitations and possibilities of using the various methodologies to obtain information about the reaction chemistry of polyphenols are discussed. Three different spectra were detected dependent on the pH and oxidizing agent. Feasible structures for the observed radicals were tested by density functional theory (DFT) calculations and the results indicate that oxidation reactions can occur at both of the catechol groups.