We report a new reaction-based approach for the detection of hydrogen peroxide (H(2)O(2)) using hyperpolarized (13)C magnetic resonance imaging ((13)C MRI) and the H(2)O(2)-mediated oxidation of α-ketoacids to carboxylic acids. (13)C-Benzoylformic acid reacts selectively with H(2)O(2) over other reactive oxygen species to generate (13)C-benzoic acid and can be hyperpolarized using dynamic nuclear polarization, providing a method for dual-frequency detection of H(2)O(2). Phantom images collected using frequency-specific imaging sequences demonstrate the efficacy of this responsive contrast agent to monitor H(2)O(2) at pre-clinical field strengths. The combination of reaction-based detection chemistry and hyperpolarized (13)C MRI provides a potentially powerful new methodology for non-invasive multi-analyte imaging in living systems.