We present an analytical method for the real-time detection of free radicals from the photodecomposition (ultraviolet radiation, k ¼ 254 nm) of hydrogen peroxide (H 2 O 2) using frequency-mixing magnetic detection. We monitored the free radicals in situ without catalysts or probe molecules. Both water and H 2 O 2 produced frequency-mixing signals under UV radiation, but the water signal was much weaker. The root mean square amplitude of the frequency-mixing signal was found to depend on the initial H 2 O 2 concentration. Considering the physical properties of the reactants, the frequency-mixing signal is attributed to the generation of paramagnetic free radicals by the photodecomposition of H 2 O 2. V