Real-time detection of gaseous hydrogen peroxide (H2O2) in ambient air is an important subject for public safety and security. We have developed a mass spectrometric detection system for gaseous H2O2 using dielectric barrier discharge as the excitation source for metastable rare gas atoms. The O-2(-) ion produced by Penning ionization of ambient air forms a stable cluster ion [H2O2 + O-2](-) which is detected by an orthogonal time-of-flight mass spectrometer. No interference by the moisture in the air was observed.