A mass spectrometric study was made of oxygen activated by microwave and by a-c. glow discharge. Appearance potential curves for normal oxygen at masses 16 and 32 indicated the occurrence of multiple electron impact processes. The change in the curves when the oxygen was activated could be interpreted by assuming the presence of O-atoms in the3Pground state, and O2molecules in the1Δgexcited state. No evidence was obtained for the presence of ozone up to pressures of 2 mm. Hg. The recombination coefficient of O-atoms on pyrex was found to be 1.1 × 10−4. Only one oxygen atom in 21 was ionized before recombining in the mass spectrometer ion source. The rate constant for the reaction of O-atoms with N2O is less than 1 × 10−8 cm.3mole−1sec.−1, and several orders of magnitude less than this for the reaction O(3P) + N2O → 2NO. The reaction of O-atoms with NO2was much faster than with NO, but no evidence was found for the formation of NO3.