The mobility constants for oxygen negative ions in He, Ne, A, Kr, Xe, H2, N2, and CO2, each containing various partial pressures of O2were measured, and extrapolations to zero partial pressure of O2were obtained. A measurement of the mobility of the negative ion in 100% oxygen was also made. The value found for the latter was 2.46±0.05 cm2/v‐sec at STP. It was shown that the mobility values referred to a single species of oxygen ion, but the question as to whether it was O2−or O3−was not resolved. A time‐of‐flight method was used, in which the ions were made by the passage of alpha particles through the gas, and detected by a proportional counter.