[en] An experimental system for on-line mass spectrometric analysis of gas phase radiolysis systems was developed. Oxygen in a fast flow reaction tube was irradiated with 1 MeV protons and subsequently sampled with a molecular beam source. Analysis of the modulated molecular beam with a quadrupole mass spectrometer provided the concentrations of O, O₂, and O₃ as functions of reaction time, dose rate, and system pressure. Observed ozone levels were anomalously high compared to the predictions of existing chemical models, with apparent g values considerably above theoretical maximum values. Agreement of theory with the data was improved by postulating an excited state of O₂ as a direct precursor to ozone