We report a new reaction-based approach for the detection of hydrogen peroxide (H2O2) using hyperpolarized C-13 magnetic resonance imaging (C-13 MRI) and the H2O2-mediated oxidation of alpha-ketoacids to carboxylic acids. C-13-Benzoylformic acid reacts selectively with H2O2 over other reactive oxygen species to generate C-13-benzoic acid and can be hyperpolarized using dynamic nuclear polarization, providing a method for dual-frequency detection of H2O2. Phantom images collected using frequency-specific imaging sequences demonstrate the efficacy of this responsive contrast agent to monitor H2O2 at pre-clinical field strengths. The combination of reaction-based detection chemistry and hyperpolarized C-13 MRI provides a potentially powerful new methodology for non-invasive multianalyte imaging in living systems.